Catherine Breillat: 'My sister's scared to see it'

Catherine Breillat's films tend to upset people. Her latest, about sex and sibling rivalry, is no exception.

Catherine Breillat is fascinated by breasts. And pubic hair. And pale, shivering inner thighs. She's not averse to showing the odd erect penis (as in her sixth, most notorious film, Romance), but it's women's bodies that obsess her, women's bodies being groped, fondled and generally screwed. The French director's new film, A ma soeur! – a tale of two sisters on family vacation, the beautiful, 15-year-old Elena and pudgy, 12-year-old Anaïs – displays acres of vulnerable female flesh, particularly where Elena is concerned. This has already caused problems in Canada – last week the film was banned in Ontario. Roxane Mesquida, the actress who plays Elena, has repeatedly been asked by the press if she felt exploited. Says a triumphant Breillat: "Is Canada Iran now? We did the sex scenes in one day – as soon as Roxane did it, she glowed – she told me it was the most beautiful day of her life!"

Ye Gods. At such moments, the 52-year-old auteur sounds like a libertarian Barbara Cartland. Yet she can also be witty and wonderfully strange. A ma soeur! – a patchy film redeemed by one of the best, most shattering endings I've seen all year – reflects the chaos. Elena has an invasive adult lover whom we're encouraged to despise, but Breillat's camera is just as predatory, guzzling on Elena's painfully skinny form. Then again, Breillat can flip all preconceptions around, especially in relation to Anaïs (played by Anaïs Reboux). Whether bolshily passing judgement on the world, weeping as she hears her gullible sister being deflowered, or facing her own attackers in a blaze of rage, Anaïs is proof that ugly sisters don't have to be horrid, and that not all little girls are looking for a prince. With her Theda Bara eyes and glorious, Marlon Brando waddle, she roots us in something real. Violently sexual, she doesn't titillate – she moves.

That A ma soeur! is largely based on Breillat's own experience comes as no surprise. And you just have to take one look at her to see which sister she is. Tucked up in a smart suit, she's a solid square of diffident aggression (imagine The Last Seduction's Linda Fiorentino impersonating Victoria Wood). She's ever so animated, staring into your eyes, gesturing wildly with her arms, yet when I mention she's got to be photographed, she goes stiff and grabs her make-up kit. She likes to look, clearly, but isn't so sure about being looked at.

Maybe that's what happens when you grow up with a beautiful older sister. A beautiful older sister ("so beautiful, like Audrey Hepburn, with big green eyes and cheekbones") who took up hardly any space. Breillat explains that her sibling was born, after a full term, weighing only a kilo and a half ("She was born anorexic!"). Breillat arrived a year later, and because she was normal, she says, she was treated like a tomboy: "Food had a big importance in our household, because my parents were always trying to get my sister to eat. I did eat and expected congratulation, but I never got it." She quickly became taller than her sister. She wasn't fat, she insists, and didn't have an odd relationship with food, yet later she talks about how she developed size-36 breasts aged 11: "They were like boulders" she says, gesturing with her arms and nodding furiously, "like a fat girl."

As a teenager living in the provinces, Catherine decided she wanted to be a singer (the haunting song Anaïs sings about death and rotting meat was made up by Breillat when she was 12). She also wanted to be an actress. At 16, she began going to casting sessions in Paris, with her "little" sister in tow. "She followed me everywhere" says Breillat, clearly still outraged. "She treated me as if I were responsible for her, it drove me crazy." Poor little Bambi, you see, always emerged the winner. As Breillat says, "When the directors looked at my sister they'd think she was more beautiful and talented than me." Her sister became a full-time actress. Breillat rolls her eyes: "So I quickly became a writer."

An X-rated one. Written when she was 17, Breillat's novel, L'homme facile, couldn't be purchased by under-18s, which meant she couldn't buy it either. From this point on, she played the devil to her sister's saint. "I was a rebel in every sense of the word," she purrs. "People who liked rebellion loved me and hated my sister, who was the image of a good, proper actress". Her sister, she says, was always trying to steal things from her – "even my husband" – but she purposely chose people who put her first. "My husband was very handsome and my sister was fascinated by him, but he got fascinated by me – that was unbearable for her!" The relationship still sounds fraught. Has Breillat's sister seen the film yet? "No, she'd rather be cross with me and not have seen the film." Big laugh. "I think she's scared to see it."

Sex'n'violence are Catherine's two trump cards; in A ma soeur!, they are Anaïs's weapons, too. In the explosive final scenes, Anaïs finds a lover who literally destroys the competition. No wonder her sister doesn't want to see it. Breillat is sanguine: "Maybe that's a weakness on my part, that the people I like have to love only me."

Breillat demands the same sort of loyalty from her performers. And the real Anaïs, bullied at school for being fat, was very eager to please. In one particular scene in A ma soeur!, we see Elena feeding a distressed Anaïs at the dinner table. It's hard to watch, because the very thing that's supposed to be comforting the little girl seems to be choking her. "Ah, yes," says Breillat, cheerfully, "I think Anaïs felt very bad during that scene – to upset people you have to go deep down into yourself to find something that upsets you. She was blinded by the tears, she was nearly asphyxiated by the food – I just stayed behind the camera saying, 'Keep eating!'."

The 18-year-old Roxane ("so, so beautiful") was also accommodating, reassuring Breillat, as we've already heard, that she couldn't have been happier about the nudity. Indeed, the pair got on so well they're currently making another film, which is all about what it's like to shoot an erotic film. "Roxane is very brave," says Breillat proudly. "When journalists ask if she's been manipulated by me, she gives them a look that could kill!"

Things weren't so simple, of course, with the star of Breillat's previous film, Romance. Caroline Ducey was 20 years old when she made the picture, with a history of eating disorders. She told the press that she felt used and abused by Breillat. Breillat mentions this without prompting: "Caroline had the same wonderful experience as Roxane at the time we made the film," she says, excitedly. "Caroline went 'Wow!', she glowed, but then she freaked out afterwards and said that what she did in the movie was my fault." She shakes her head. "It was a lie, they were lies, and it's very hard for me to forget them."

She can't envisage working with Ducey again. She says sternly, "I no longer dream of looking at Caroline. If you can't assume responsibility for what you do, then you're not an actress."

An unwillingness to take responsibility... Breillat homed in on that very same fault in her sister. It seems to be the thing she hates most – little girls lost who bring out the protective instinct in others and make Breillat look like a bruiser. Then again, maybe she just likes a good fight. As I'm leaving, it suddenly occurs to me to ask if she's seen Michael Haneke's The Piano Teacher, another film about sex, love and degradation. She hasn't seen it. I think you'd like it, I say. She smiles vaguely, then snaps to attention: "I'm not so sure."

We shake hands, then she marches off, grim-faced, to have her picture taken. On my way home, I see Roxane, who's also over doing publicity, being photographed in the street. The photographers are running through traffic, risking life and limb to get the perfect shot. "Smile!" says one, as Roxane wearily recrosses her arms. She's the one they all want to look at – the skinny little beauty – but this time she's on Breillat's side. How long, you wonder, will this romance last.

'A ma soeur!' opens on 7 December