Miu Miu, the little sister to Italian fashion house Prada, has grown up.

After living in Prada's shadow for years, the label is finding that its moves to carve out an independent identity have paid off, catapulting Miu Miu into the top league of Europe's fashion scene. Sales at Miu Miu, which unveils its spring/summer 2011 collection Wednesday in its largest venue yet, have more than doubled in the past four years.

Prada designer Miuccia Prada founded the label, which takes its name from her nickname, in 1993. Four years ago, Miu Miu decamped from Milan to Paris to stage its biannual fashion show, creating a new showcase for its distinct style. Its frills and glitz presented a sharp contrast with Prada's minimalism.

"It's feminine and sexy but modern too," wrote Lorraine Candy, editor-in-chief of Elle fashion magazine's U.K. edition, which together with three other fashion magazines put the same Miu Miu look—a dress with flowers on the straps and a bow across the front—on their August cover pages.

Miu Miu's revamp is important to the future of Prada SpA, which owns shoe brands Car Shoe and Church's in addition to Miu Miu and Prada. Prada's owners—Ms. Prada and her husband Patrizio Bertelli, who is chief executive—hope to list Prada on the stock market in the coming years. Analysts say the group would make a more compelling investment if it didn't rely only on the Prada label for nearly all of its sales growth. Of the company's $1.3 billion in first-half sales, $220 million was generated by Miu Miu, which grew 40% during the period.

For three decades, many fashion houses have created less-expensive versions of their main collections as a way to expand their businesses. Giorgio Armani launched Emporio Armani and Armani Collezioni for consumers who couldn't afford the high price tags of his main line. Dolce & Gabbana created D&G as a younger and edgier variation of the original brand. Just Cavalli is the younger sibling of the Roberto Cavalli brand.

Prada has tried to do something different with Miu Miu. Though the brand is designed by Ms. Prada, the company's executives have gone to great lengths to fight the perception that Miu Miu is simply a more wearable and affordable spin-off of its bigger sister brand.

Miu Miu's inaugural Paris runway show in 2006 featured clothes in expensive fabrics and elaborate techniques, such as hand-painted brocades and printed silks. Provocatively flirty skirts replaced the simple shift dresses and dowdy sweaters that Ms. Prada had previously designed for Miu Miu. For the brand's debut Paris show, Ms. Prada brought tailors with her from Italy to Paris to put the final touches on the ensembles. Trendier than Prada, the label is popular among celebrities, such as Carey Mulligan and Keira Knightley.

While Miu Miu still generally costs less than the Prada label, in recent years, the company has pushed Miu Miu's prices high. A pair of cream and black patent pumps sell for $650, compared with $550 for Prada's classic black heels.

"Miu Miu and Prada do not cannibalize each other. They are friendly competitors," said Prada Chief Operating Officer Sebastian Suhl in a recent interview. When a Miu Miu store opens near a Prada store, Prada's sales don't suffer, Mr. Suhl said.

Robert Burke, a luxury-goods industry consultant, says that Miu Miu's recent success has also been helped by a change in consumers' dressing habits. Years ago, when consumers dressed head-to-toe in one designer label, cheaper, secondary lines allowed consumers who couldn't afford a top designer line to still buy into a designer's look. "Today people don't shop that way. They are just as happy to shop across different categories and brands," Mr. Burke says.

Miu Miu has also revamped its Milan flagship store in damask and rich colors—a change from a more minimalist concrete look—and has redecorated a network of 75 boutiques around the world.

Mr. Suhl said that he wants Miu Miu to be sold only in U.S. department stores that will give the brand its own in-store boutique, rather than putting it out on the floor with other brands. It is rolling out more in-store boutiques in U.S. department stores. It has long had its own boutique at Bergdorf Goodman and plans to open one at Bloomingdale's this month. Next year, Miu Miu will have 20 such "shop-in-shops" in U.S. department stores, displaying a full array of clothing, shoes and bags.