Sienna Miller

Danny Martindale/WireImage

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

Peter Kramer/AP

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

Splash News Online(2), Startraks(2), Splash News Online

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?

Flynet

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

Neil Rasmus/PatrickMcMullan/Sipa

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?

Gareth Cattermole/Getty

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.”

CAROLINE TOREM CRAIG/LFI

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?