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It was a family affair at the Twenty8Twelve show on Monday night when Sienna and Savannah Miller’s mom, Jo Miller, step-mother Kelly Hoppen and Savannah’s husband took their front row seats at the sisters’ second London Fashion Week collection. Also in the front row — Anna Wintour and MTV presenter Alexa Chung. Backstage, the design duo confessed to PEOPLE that the collection had been hard work and was the product of much sisterly bonding and midnight texting. “We do finish each others sentences,” Savannah told PEOPLE. “Yeah, I guess we are really lucky to have a great relationship where we can work together,” agreed Sienna, as they celebrated with a much-needed cocktail. And with Sienna currently performing in Broadway’s After Miss Julie across the ocean in New York City, the close-knit sisters have proved to be as creative about keeping in touch as they were for their collection of tight denim dresses and striped leotards. “Usually I get my best ideas in the middle of the night,” said Sienna. “We text all the time and we’re always on the bbm [BlackBerry Mobile],” added her sister. With that, the actress fled the backstage area to board a plane back to New York with her mom, her dog and the memories of Anna Wintour applauding her very own fashion moment.–Monique Jessen

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The Devil Wears Prada and so did Anna Wintour to the premiere of “The September Issue” last night at the Museum of Modern Art in New York, but according to designers and family alike at the premiere, that’s where the similarities end. As Wintour was joined by Renee Zellweger, Sienna Miller (who appears in the documentary as the cover girl for the titular September 2007 issue), Carolina Hererra, Oscar de la Renta, Donna Karan and more fashion glitterati, friends and relatives described Wintour as anything but a Devil. “She’s an amazing mother, an absolutely incredible mother,” says Wintour’s equally-stylish daughter Bee Schaffer. “She came to all my sports games. Her kids are the most important thing to her.” But that doesn’t mean that Mom doesn’t offer unsolicited fashion advice, like what to wear on a date. “She told me to change” for a recent night out, Shaffer revealed — although she stuck to the original outfit. And Wintour doesn’t just give advice to her children. Designer Zac Posen recalls Wintour once “asked me to take a sleeve off of a dress” early in his career. “I refused,” said Posen. “And a common respect level was brought to the table. And then the dress was later shot on Debra Messing.” Designer Marc Jacobs also described a formidable Wintour with a heart. “She’s smart and she surrounds herself with the most talented people. And she’s a risk-taker unafraid of taking chances. She doesn’t suck up,” he said. “And she’s been very good to me on a personal level as well. When I was having some personal problems, Anna was always there for me. She’s really a great editor. And she’s a good mother. She’s very maternal and caring to people she cares about. Anna’s really quite a warm person, who maybe has a different reputation. But in real life she’s very warm. . . and generous.” And while director R.J. Cutler received “extensive notes,” from Wintour, the editor tells PEOPLE “He ignored every single one. It’s his movie . . . his vision.” And does Wintour, who has seen the film twice approve? “I think it’s a wonderful document, on the amazing hard work, the creativity, the talent, the heartaches and the success.” The September Issue opens in N.Y.C on August 28 and nationwide on September 11. – Jeffrey Slonim

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No one has ever doubted Sienna Miller’s commitment to fashion — from cutting edge style on the red carpet to even co-designing her own Twenty8Twelve line with her sister Savannah — but she’s taken it to a whole new level this week while promoting G.I. Joe: Rise of the Cobra in N.Y.C. Yesterday, the tireless fashionista changed five times, with each outfit more glamorous than the next! Her day kicked off with a plunging red frock paired with bow-topped Chloe sandals, switching into a shimmering top and black wrap skirt from Derek Lam. Next up, a confetti print Diane von Furstenberg dress with sharp shoulders paired with two-tone T-straps. The evening saw a darkly printed minidress paired with her favorite studded black booties before changing into a casually chic black Diane von Furstenberg jumpsuit and gladiator sandals for dinner at the Waverly Inn. Phew, we’re exhausted just thinking about wearing so many gorgeous looks in one day! Tell us: Which of Sienna’s looks do you like best?

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Sparking an all-out fashion frenzy, stylish stars turned out in London last night to help designer Matthew Williamson celebrate his collaboration with H&M. The British designer, who kept pal Sienna Miller close by his side, told PEOPLE just how nervous he had been while heading to the launch of his capsule collection with the cheap-chic retailer: “In the car on the way I said to Sienna, do the heart test, feel my heart, and she did. It was pounding!” But it turns out Williamson had nothing to worry about. As VIPs like Mischa Barton and Pixie Geldof grabbed his signature vivid color pieces off the rails of the Regent Street store, he looked on with disbelief. “I was so nervous that no one was going to be here. I’m watching the rails being emptied, literally, it’s freaking me out — in a good way!” For her part, Mischa seemed to agree, congratulating the designer with a huge hug as she arrived in one of his new designs — a studded leather waistcoat and blue tulip skirt. “This is amazing,” she said pointing to a cropped leather jacket, while Sienna Miller, having completed her mission of support, sipped champagne quietly with a few friends. You can finally nab some of Williamson’s must-have looks for yourself. His collection, which ranges from $25 to $349, launchs today in H&M stores. –Reporting by Monique Jessen

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She may have her own fan-base of ultra-glam celebrities, but star stylist Rachel Zoe is making no secret of her love of the “free spirit approach” of star designer Sienna Miller’s ready-to-wear line Twenty8Twelve, created in partnership with her sister Savannah Miller. Zoe had the rare opportunity to interview the designing duo for Interview magazine — and a love of fashion, she learned, was a family affair from the start. “Our mum was very glamorous in a shimmery, ’80s sort of way,” Sienna says. “She had endless dinner parties. It was very Bohemian and inspiring to grow up around creative people in that way.” And when Savannah headed to the esteemed St. Martins design school, Sienna naturally offered input on her sister’s projects. “When we were approached to do the line,” Sienna reveals of the collection’s 2006 launch, “it felt like an organic step to take. It means we get to spend time together, and it’s a lot of fun.” And for her part, interviewer Zoe wouldn’t mind stepping into the sisters’ shoes, telling them, “It would be my dream to form a business with my sister and I envy you both for doing so.” Click here to read Zoe’s full interview with Sienna and Savannah. Then tell us: What do you think of their designs? Would you want to work with your sister?

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While most of Hollywood was in Oscar mode, Sienna Miller and her sister Savannah were making their runway debut at London Fashion week. The designing duo behind Twenty8Twelve debuted their Fall 09 collection to a packed house which included Fashion Week staple Kanye West and fellow British designer Matthew Williamson. The clothes seem a combination of the two sisters; leather jackets, skin-tight dresses with a boho flair for Sienna mixed with crystal-embellished cashmere, fine-tailoring and florals for Savannah. “But put them together, and it says London cool,” Sienna tells the The Telegraph backstage before the show. “These clothes are funky, and there’s something for anyone.” Or, as Savannah tells Style.com the collection is “Pirates of the road meets Grace Jones.” But even with the success of the two-year old line, don’t expect Sienna to give up her day-job any time soon. “I’m not giving up acting, I love acting,” says Sienna. “But I also love working with my sister. We’ve always been two sisters playing dress-up, ever since we were kids and we’re still doing it.”

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Once you have a successful film career, high-profile relationships and a successful clothing line, what’s left to do? Have a runway show, of course! Sienna Miller and sister Savannah are gearing up to show their popular fashion line Twenty8Twelve at London’s Fashion Week come Febraury. The presentation of the Fall/Winter 2009 will be the first time that the Miller sisters have taken their line of feminine dresses, tailored jackets and edgy accessories to the catwalk, and we’re already wondering what star pals we might spot in that front row! Watch out, Olsens — will Sienna and Savannah be next year’s designers of the year?

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Sienna Miller and Keira Knightley are both known for their quirky British style — so what happens when you cast them both in the same film? Not only do they become close friends, but apparently they coordinate their red carpet outfits! The stars spent their day promoting The Edge of Love at the Edinburgh Film Festival. First, they hit a photocall in matching booties and black accents. But they hit the nighttime premiere in ’30s-inspired black and white — Sienna in long fringe and a bowler hat, Keira in a Chanel jumpsuit and bolero. We can’t wait to see if the actresses keep up their red carpet team effort. Tell us: What do you think of Sienna and Keira’s paired-up looks?

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Our closets are jam-packed with frocks in pretty much every color of the rainbow–but when in doubt, we always go with a little black dress. And we’re not alone in relying on the style staple. Just about every celeb in Hollywood rocks the LBD from time to time, just like Sienna Miller did in a this lace Monique Lhuillier number for the Film Independent’s Spirit Awards. Click over to our friends at FabSugar for great tips on how to choose the most flattering LBD silhouette—and learn other insider tricks for keeping the classic look fresh and fabulous.

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Scarves couldn’t be hotter for fall — and what better way to add a touch of the wild to any outfit than with a dash of leopard-print? Whether you wear it with neutrals like Keira Knightley, bold colors like Sienna Miller or with dresses like Eva Longoria, this is a can’t miss accessory! Here are a few of our favorites for you to try out. Tell us: Is this a trend you’d try?

Vote on even more star trends by clicking here!

Kashmere Camel Leopard Printed Cashmere Wrap, $135 at bluefly.com

VERA Leopard Crepe De Chine Scarf, $62 at ronherman.com

Echo Leopard Print Scarf, $38 at bloomingdales.com

Animal Multi Purpose Scarf, $9 at asos.com

Forever 21 Leopard Scarf, $7 at forever21.com

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Though we’ve seen lots of stars go dark for roles in the past few months — Jessica Biel and Mischa Barton pop to mind — we had to look twice when Sienna Miller appeared with long brunette hair on the set of her new film Hippie Hippie Shake. She was almost unrecognizable with her wig on! We definitely prefer the British star with her normal blonde bob that highlights her big blue eyes and unique fashion sense, but we want to know what you think. Tell us: Do you like Sienna with brunette hair? Or should she stick to blonde off camera?

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Looking like she fell out of the 1950’s in a red bodysuit and leopard scarf, Sienna Miller celebrated her clothing line with sister Savannah in an intimate party at Bergdorf Goodman on Wednesday night. And lest you worry that Twenty8Twelve by s. miller is only for willowy stars like Sienna herself, she assures PEOPLE that’s not the case. “Throughout the collection we’ve really covered all different ages, all different body types. It’s really fashionable clothes for all different sorts of confident women.” Speaking of confident women, we had to know if there was any sibling rivalry involved in the design process. Savannah, the older and taller of the sisters, tells PEOPLE that “We both bring something very different to the line, and working together we complement each other. We get along very well. We know each other very well so it’s never a conflict. It’s a really good time to spend time together doing something we both love so it’s great.” Sounds like a perfect match! Shop the collection at my-wardrobe.com.

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Vests made a comeback last fall — and from menswear-inspired to open cropped styles, we thought we’d seen them all. But then some fashion-forward stars have come up with a new twist on the wardrobe staple — cropped, multi-colored vests. From Mischa Barton’s all-sequined number to Jessica Simpson’s bright macrame version to Sienna Miller’s metallic patchwork, the trend seems to be taking hold of some pretty stylish women! Besides the cropped length — which is tough for most women to pull off — there is something about these that seems more costume-y than retro-cool. But maybe that’s just us! Tell us: Would you wear these multi-colored vests? Is this a trend you want to try?

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In the August issue of StyleWatch, you can check out stars like Cameron Diaz, Rihanna, Kylie Minogue, and more sporting the latest accessory du jour — glossy handbags. Sienna Miller was recently spotted on her way to the airport carrying this chic Chloe bag — which adds just the right polish to her casual travel outfit. For a classic look, go with darker colors, like black, navy or chocolate brown, or go bold with metallics or bright colors. Below are some of our favorite picks. Tell us: Would you wear a patent leather bag?

Get the Look!
Splurge:
Burberry “Beaton” hobo, $850, nordstrom.com

Michael Michael Kors “Chestertown” satchel, $398, nordstrom.com

Adrienne Vittadini “Norena” Tote, $228, dillards.com

Steal:
“Natasha” patent tote, $50, baghaus.com

Patent tote, $40, gap.com

Zou Zou quilted patent messenger tote, $20, jcpenny.com.

And be sure grab the brand new August issue of StyleWatch, on sale now!

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We’ve talked before about what a difference a tent dress makes for a woman’s figure, as in totally obscures it. Since we’re pretty used to seeing Sienna Miller in floaty frocks, we didn’t think much of it when she arrived at the Late Show in a babydoll gray dress from her new line Twenty8Twelve. But our jaws dropped when she emerged from taping the show in a second-skin black Calvin Klein sheath dress and sexy high heels — va-va-voom! Who knew that body was hiding under all those layers? We totally relate to Sienna’s love of loose dresses, but we can’t help but wonder if she shouldn’t give them a break. Tell us: Which dress do you like better on Sienna? Should she stick to form-fitting dresses?

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As the weather gets hotter, we just want our clothes to get easier — and what’s simpler than a dress than can go day to night and requires the bare minimum of accessories? We’ve spotted Eva Mendes, Nicole Richie and Sienna Miller wearing smocked tube-dresses with great bold patterns and long sweeping hems. Just toss on a few bangles, a cool bag and comfy sandals, and you’re set! Dress it up for night time with jeweled flip-flops and an oversize clutch. The best part of these dresses is the smocking — unlike normal strapless dresses that tend to fall, these stay snugly on the bustline. Here are a few of our favorites. . .Tell us: Would you try a smocked sundress?

Get Eva’s Juicy Couture Dress, $198 at neimanmarcus.com.

Tropical Tube Dress, $23 at forever21.com

Mossimo Long Floral Halter Dress, $22 at target.com

L8er Smocked Strapless Floral Maxi Dress, $45 at macys.com

Roxy ‘Iris’ Print Tube Dress, $50 at nordstrom.com

Photo: splash news online; Richard Young/startraks

Sienna Miller has become known for being a fashion icon over the past few years, so it only makes sense that she would join the ranks of Sarah Jessica Parker, Kate Moss and the Olsen sisters in designing her own line. The added bonus is that her sister, Savannah, is actually a graduate of Central Saint Martins school of design in London and a fashion designer! The sisters have paired up to create Twenty8Twelve and instead of rolling it out with a big splashy fashion show, Sienna’s just started wearing the clothes — and who could be a better model? She wore her elegant little black dress to the recent Glamour Women of the Year Awards and her Gabrielle cotton camisole paired with jeans while strolling through London. We’ve got to say — so far, so good! We can’t wait to see the rest of the line in the fall. Tell us: Would you wear Sienna Miller’s clothing line?

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Sienna Miller has been super-blonde verging on platinum for quite a while, due in no small part to her role as Edie Sedgwick in Factory Girl. But now for another role, this time in The Edge of Love with Keira Knightley, the actress has dyed her hair a dark caramel. We think Sienna is one of those rare lucky women who can probably pull off any hair color, but we want to know what you think! Tell us: Which color do you like better on Sienna? Platinum or caramel?

Click here to vote on more celeb transformations in the What’s Her Best Look Gallery!

Whether on the red carpet or running errands in L.A., Chanel has always been a favorite of Hollywood’s “it” girls. Mary-Kate Olsen even wrote an ode to her classic red Chanel bag. This season, stylish starlets are scooping up the Chanel 2.55 handbag in patent calfskin, white, black or ruthenium metal. Like all things Chanel, the flap bag doesn’t exactly come cheap. Sold exclusively at select Chanel boutiques, it carries a hefty price tag– $2675 to be exact (call 800-550-0005 to order). Luckily, you can get the look at SophiasReplica.com for a more reasonable $120. Tell us: Would you ever splurge on a real Chanel bag?

Photo: Sylvain Gaboury/FilmMagic; Earl/Field/Splash news online; FAME

Even though she got her printed hoodie as a freebie at the Sundance Film Festival, Sienna Miller proudly shows her support for the Humane Society’s Protect the Seals Campaign. The clever slogan “Club Sandwiches Not Seals” is meant to bring attention to the many seals that are clubbed to death every year in Canada. Printed on sweatshirts like Sienna’s and polo shirts, the Humane Society uses 100% of the profits to raise awareness about the cruelty facing Canada’s seals. For Sienna’s sweatshirt, $50, click here and for other seal campaign t-shirts, including the new “Seal the Deal” campaign logos, click here. To find out more about Canada’s cruel seal hunt visit protectseals.org. Tell us: Would you wear this sweatshirt?

Photo: ATR/FINALPIXX