When Jennifer Weiner attends the premiere of “In Her Shoes” — based on her 2002 chick-lit bestseller — she’ll wear a brand new hairdo.

This past summer, both the 35-year-old and her younger sister, Molly, an actress, had identical geometric bobs. But since her sister is one of her dates for the premiere, Weiner grew her brown hair shoulder length and added blond highlights and loose waves she says are very “in” for fall.

“I decided we couldn’t both have the same hair on the red carpet,” she adds with a laugh.

The way sisters compete and relate is the subject of Weiner’s novel and the movie, adapted into a screenplay by Susannah Grant (“Erin Brockovich”) and directed by Curtis Hanson (“8 Mile”). Like the book, the droll but heartfelt film revolves around Jewish siblings who have nothing in common except size 8 1/2 feet and a wicked stepmother. The fictional Maggie (Cameron Diaz), a size 0 babe, is an irresponsible party girl with dyslexia. Rose (Toni Collette), a frumpy size 14, is a successful attorney with low self-esteem. Rose collects shoes to make herself feel better; Maggie covets and pilfers the boots and high heels.

It is only when the sisters reconnect with their long-lost grandmother (played by Shirley MacLaine) that they learn to make peace with each other — and their footwear. The shoes become a metaphor for all the ways the sisters are jealous of each other — “for wanting to inhabit someone else’s skin and get what they get out of life,” Weiner says.

As she wrote “In Her Shoes,” she wanted to work through an obvious, but puzzling, conundrum: How can people who grew up in the same house wind up radically different individuals?

The author is barefoot in her Philadelphia bedroom after her recent haircut, and insists that the fictional siblings are not versions of herself and her own sister. But their relationship did spark questions that inspired the story. Weiner’s sister didn’t steal her shoes, but she took her clothes, mostly her plus-sized sweatshirts, when the 1980s film “Flashdance” made oversized sweats fashionable.

The dark-haired Weiner “felt like an outsider in so many ways,” and says she was “funny-looking, brace-faced and plump.” On her youth trip to Israel, where there were four other Jennifers, she was labeled “the fat one.”

Weiner spent the next decade dieting and seeing nutritionists — until she had an epiphany in the late 1990s. “It had been 10 solid years of trying to get somewhere my body didn’t want to go,” she says. “And I really just got to the point where I thought, ‘How much more nonsense am I going to put myself through, and how much time am I going to waste? And looking at the world and seeing the genuine suffering and injustice, how much more of my life do I want to devote to looking like Jennifer Aniston?’ And I said, ‘I am through with this, and I’m going to work with what I have and try to be happy and take some of this energy and put it someplace else.'”

The energy went into writing her semiautobiographical debut novel, “Good In Bed,” whose troubled, zaftig heroine winds up living happily ever after without shedding a pound. That’s more or less what happened to Weiner, who is now married to a menschy attorney, has a 2-year-old daughter and a stellar writing career to boot.

Her wickedly witty, but flawed, heroines have made her the biggest chick-lit success story since Helen Fielding burst through with “Bridget Jones’s Diary,” according to Entertainment Weekly. The Jerusalem Post called Weiner the Jewish girl’s answer to Fielding. Weiner says she enjoys creating Jewish, plus-sized heroines, partly because she is writing what she knows, and partly because such characters are often invisible in popular culture. Heavy women, especially, are ignored or played for laughs.

Considering Hollywood’s weight phobia, Weiner felt victorious when actress Toni Collette (“The Sixth Sense”) agreed to gain 25 pounds for the film. Collette has admitted she was reluctant to put on the weight. “But I love my character and I think the extra pounds are pertinent to the way Rose sees herself,” she said at a press conference.

The movie also features amusing, if occasionally clichéd, Jewish characters based on residents of the retirement community where Weiner’s grandmother lives. There is a joyous Jewish wedding and a grotesque caricature of the stereotypical Jewish American Princess in the sisters’ wicked stepmother. Weiner — who finds the character “recognizable” — loves the scene in which the “stepmonster” gets her just desserts. (She discovers that her biological daughter has joined Jews for Jesus.)

If the fictional sisters enjoy their Cinderella-like happy ending, so does Weiner. Her books have sold millions of copies worldwide; Hollywood is snatching up the movie rights, and her latest novel, “Goodnight Nobody” just hit bookshelves.

And then there’s the “Shoes” premiere, where the author will sashay down the red carpet not in glass slippers, but in strappy Nine West silver stilettos. Perhaps she’s hoping her sister will wear something else.

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