Sienna Miller

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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