Sienna Miller in Twenty8Twelve

Sienna and Savannah Miller are two siblings that have definitely learned to share. The stylish sisters-and former roommates-co-design a line that ranges from frothy dresses to lean and mean jeans. A recipe for rivalry? Not a chance. "Sienna's edgy and out there; I'm soft and romantic," Savannah told a U.K. newspaper. "I have an idea for a design and she adds the sex and flips it on its head. It's fun and it works."

BUY ONLINE NOW
Cropped jacket, $279; at intermixonline.com.

If you want to see some killer shoes-like the elfin booties Sienna wore with the label's striped dress-stop in the sisters' London shop. "I quite like the idea of being at the till in my Louboutins," Sienna has said.

BUY ONLINE NOW
Gastan tank dress, $228; at ronherman.com.

Savannah had a convenient muse in sister Sienna while studying at Central St. Martin's design school. "I had always used Sienna as my fit model," Savannah told WWD. "She would also always give advice on how things should look. She's always had this great sense of style."

BUY ONLINE NOW
Silk dress, $181.90; at saksfifthavenue.com.
Cropped jacket, $279; at intermixonline.com.

Savvy stylesetter that she is, Sienna has turned the red carpet into her personal fashion show. She debuted this full-skirted dress at an awards ceremony last June. And she's also put the paparazzi to work for her; the oft-photographed star hits the streets in her favorite prototypes.

Sienna was spotted in a not-so-exclusive tailored tuxedo jacket: sister Savannah wore the look months before at the London premiere of Factory Girl.

BUY ONLINE NOW
Cropped jacket, $279; at intermixonline.com.

It's no mistake that Sienna's empire-waist dress fit her to perfection. "We really look at each piece, its tailoring and materials," she told WWD.

The sisters drew inspiration for their debut collection (called "An Anthology of Rebellion") from Victorian London, 70's rocker style and vintage pieces Sienna purchased from Decades. The influence was evident in Sienna's ruffled tunic worn with sleek leggings.

Sienna accessorized a shirt from the line with her tried-and-true accessories. "I dress for myself and I don't care what people think," Sienna told the U.K. Telegraph. "It's dangerous when you start dressing just for effect or for the photographers. Where's the fun in that?"

Sienna kept her cool during a visit to an N.Y.C. museum in shorts from the line. Perhaps she was seeking inspiration? The art-loving sisters host rotating art exhibits in their shop in London's Notting Hill. They also commissioned artist David Cooper to design prints for their debut collection.

A perfect summer look-the line's classic indigo shift dress. "Our brand is not about setting trends," Sienna has said.

BUY ONLINE NOW
Chambray tunic dress, $286; at revolveclothing.com.

Sister Savannah would be proud of the simple way Sienna wore the label's crisp tunic. "The innocence of white lends itself nicely to sculptural effects," Savannah told In Style.