This is a story of two sisters. The elder is one of France's most talented living actresses, holder of two César awards and star of some of the country's most acclaimed films of the past 25 years. The younger, her junior by just 18 months, is handicapped by severe autism and lives in a sheltered residential home in the southwest of France. She is overweight, anxious, prone to violence against herself and others. Although many years ago, the similarities between the sisters were undeniable, now in almost every aspect they couldn't be more different.

Until recently, no one but family and close friends knew anything about Sandrine Bonnaire's sibling Sabine. Now, though, Sandrine has made a documentary, My Name is Sabine, about her sister's life. It is her directorial debut, and the film has become a surprise box-office hit in France. It won the International Film Critics Federation Prize at Cannes last year; the federation said it was 'the most beautiful film Cannes has given us this year'. And at this year's festival, Sandrine was on the Cannes stage again, this time accepting the Prix France Culture Cinéma 2008.

Beautiful the film may be, but it is at moments also unbearably painful to watch. Using archive video footage, a restrained commentary and family photographs, Sandrine contrasts Sabine's vibrant, if highly eccentric, youthful persona with the collapsed, neurotic 40-year-old woman we see today. A woman who keeps her dolls in a padlocked wicker chest lest she 'throw them in the potty'; who is so worried about whether her sister will return the following day she finds it hard to speak of much else; who curses and hits others, and who bites her hand in frustration, and yet who is also - as a result of the care of the therapists she's received for the past eight years - gradually growing in confidence.

It is around this issue, the care available to people like Sabine, that the film pivots. For, as well as being a harrowing portrait of a woman Sandrine describes as 'a heroine, who reminds one of someone like Camille Claudel, someone with a big but tragic destiny', My Name is Sabine is also a campaigning film, aimed directly at the French government. In France, there are no publicly funded small-scale facilities for people over the age of 20 with disabilities such as Sabine's who, in Sandrine's words, 'are not ill, but handicapped... They need places to live and to be with people who take care of them. In hospital, there are so many patients, and the doctors don't have time to take care of people, so they put medicine in them, and that's not the solution.'

Thanks to Sandrine's fame, and her provocative film, change finally looks possible. The director argued her case with President Nicolas Sarkozy; and recently, Xavier Bertrand, France's minister for work and social affairs, visited the home where Sabine lives. 'I am being invited to all the political meetings, with the right people. So yes,' Sandrine says, 'I feel hopeful.'

Such is the hectic state of Sandrine's schedule that carving out time for an interview has been a struggle, but once the moment arrives and we're seated in the hush of one of the meeting rooms in her agent's central Paris office, she is thoughtful and candid about matters which are still, clearly, a source of pain. But she's also self-evidently a movie star: her smile - which she's flashed at cinemagoers ever since her debut in 1983's To Our Loves - is as magnetic as ever. Since then, she's worked with Gérard Depardieu, Isabelle Huppert, Catherine Deneuve and Daniel Auteuil. And she's just finished filming Joueuse with Kevin Kline, in which she plays a woman who finds meaning through learning to play chess. Somehow, amid all this - she's made more than 40 films - she's managed to raise a family. She has a daughter, 14-year-old Jeanne, with William Hurt, and a three-year-old girl, Adèle, with screenwriter husband Guillaume Laurant.

Sandrine and Sabine grew up in a large, working-class family in the outskirts of Paris. Their mother was a Jehovah's Witness whose strict adherence to the sect's rules on birth control explains the number of children: 11 in total, of which Sandrine, now 41, is the sixth, Sabine the seventh. Growing up in a Jehovah's Witness home was 'quite heavy', says Sandrine. 'First of all, it was very boring. You don't do birthdays and Christmas when everyone else does them. You can have them, but three or four days after the date, so you feel apart from your friends.'

As a child, Sandrine remembers, Sabine was 'very happy, smiling all the time'. Though 'different' - prone to talking to herself and making strange, repetitive movements - she was also intuitive and insightful about others. 'She was teasing, malicious, but in a good way. She had passions for so many things. And she was intelligent, very curious, and she loved to speak and learn English.' She was musical, too: in a poignant moment in the film, Sandrine juxtaposes a video of a buoyant Sabine playing the piano as a child with footage of her at the keyboard today - she slumps at the stool, her fingerwork becoming increasingly slurred until she stutters to a halt.

Sabine began her education at a special school, but at 11 was felt to be doing well enough to transfer to the local secondary school where her elder siblings were pupils. Before long she began to be bullied. 'When we saw people mocking her, it was terrible,' Sandrine says, 'and at the same time we felt ashamed. This was our sister, and we had to protect her, but we also didn't want to admit this was our sister.' Sandrine remembers the children's taunts: 'Sabine la folle, Sabine the crazy girl.' And Sabine would answer: 'I'm not crazy.'

Her confidence, though, was slipping fast, and when she was 12 her parents took her out of school. From time to time they hired tutors for her. She carried on playing the piano. In 1983, Sandrine - 16 years old and midway through filming To Our Loves - returned from a trip to London with a young American, Tom. 'He stayed for two or three days. Sabine was fascinated with him. Then he left and went to the south of France, where someone stole all his money, which meant he couldn't go back to the States.' Tom returned to the Bonnaire home, stayed for six months, and he and Sandrine began a relationship. Sabine, undeterred, 'fell in love with him. And ever since then she was saying: "I want to go to America, I want to go to America."'

For most of that decade, however, Sabine went nowhere. While her brothers and sisters left home one after the other, she remained. Sandrine and her siblings, acutely aware how distressed these partings made Sabine, made every effort to take her on trips at weekends. 'She had no social life, no one apart from my mother. So we came almost every weekend to see her or to take her to do things. I wasn't the only one who did this. It's the kind of situation when you're happy to be part of a big family. We took her to so many places, she did so many things with all of us. But in the end, when my youngest brother left home, she was completely alone.'

In June 1993, Sandrine made her sister's American dream a reality, and the two of them flew to New York on Concorde. Sandrine recorded the trip on video: Sabine is sparky, bright-eyed, fascinated by the fact they're going back in time, telling her sister she's going to keep her watch on French time so that she can go to bed at 5pm. Near the end of Sandrine's film, Sabine sits down to watch the video of her trip to America.

Will she recognise her younger self? Will she realise what she's become? The video starts, and it's clear Sabine remembers everything. She starts moaning, her mouth open. 'It's making me cry tears of joy,' she says.

Autism is a controversial condition, evoking fierce arguments from either end of the nature/nurture spectrum. Sandrine says her mother has always suffered from feelings of guilt. 'She didn't want to have Sabine, and when she was pregnant she took some [of the anti-malarial drug] Nivaquine. Her friend had told her that if you take this when you are pregnant, the baby will go away. Of course it didn't work, but she feels guilty about it. She feels maybe Sabine is like that because she took those pills.'

As for herself, Sandrine says she's never felt guilty about the discrepancy between her success and her sister's struggles. 'The big difference is not between Sabine and me, but between all my brothers and sisters and me. They didn't go to school very far, and many of them have boring jobs.' Whatever the tensions, the Bonnaires looked after Sabine as well as they could. If she was violent, Sandrine says, it was only occasionally and only towards herself. The 'switch', however, came in 1996. Sandrine and her eldest brother Patrice were at home with their mother, and Patrice, then 40, had a heart attack. 'Sabine saw it all. By chance another of our brothers was there, and he called the ambulance, but Patrice died three hours later in hospital. This was the switch for Sabine's violence. She started hitting my mother.'

Sabine's brothers and sisters did their best to get life back to normal for her. 'We tried to have weekends with her to begin with, to make good times with her, because we knew that it was the death of Patrice and the separations from us that were distressing her. But she started getting violent with us as well as with our mother.' Her behaviour began scaring her nephews and nieces. 'Sabine never hit our children, but my daughter Jeanne saw the violence between Sabine and me and was very disturbed by it. It was clear we couldn't keep Sabine at home any longer.'

The Bonnaires turned to their local hospital for help. 'They kept her for 15 days, gave her some medicine, and after that she was the same as before, except that she was...' Sandrine pauses, lets her mouth hang open and her eyes roll back, 'drugged.'

Sandrine's manner shifts. Describing Sabine's childhood, she has been full of laughter, but now, as she details Sabine's decline, there's a steeliness to her.

'After Sabine got out of hospital, my older sister took her home for a few months. Sabine was violent again, so it was back to hospital, back to home. And after that, Sabine stayed a year and a half in hospital. She received very bad treatment. She was tied with her hands, put in a straitjacket. She had a lot of medicine.' In the end, determined to prevent any further deterioration, Sandrine rented an apartment across the road from her own flat in Vincennes, hired two nurses and arranged Sabine's release from hospital.

The arrangement didn't last long. 'After a few months, the nurses became discouraged. They were very nice, very gentle, but when patients were violent they didn't know what to do. And I could understand it; we all felt the same. And so she went back to hospital, and there she stayed for another four years.'

This time, the treatment was even harsher than before. For a long period, she was kept in isolation. 'They did this because she was headbutting, but she was only doing that because she was locked in her room. She wanted to go outside, but they didn't even allow her to go to the toilet, and there was no toilet in her room. The doctors said to us: "This is the illness, this is normal", and we said: "But she was not doing this before", and they said: "But this is normal."'

Then the hospital banned the family from visiting. 'They said we were a problem for Sabine, so they stopped our visits. And it was terrible. I have a memory, it's like a nightmare, of saying goodbye to her. There were doors with glass windows, four, five, six of them, and she went to the first door, they locked it, she was crying, and each time we went through another door, there she was at the first door, watching us. It was terrible for her - and for us.'

By the end of her incarceration, Sabine was completely 'extinguished', says Sandrine. 'She was helpless, incontinent, 30kg heavier. She trembled and drooled. Her memory was nearly gone and she could no longer wash herself. All that time, we wanted to take her away, but we couldn't find anywhere suitable. Some places had room, but they didn't have a medical service, which we needed.' Finally, Sandrine found a suitable home, but it was full. Its director, however, was looking to start up another. They joined forces, and with 'my fame and his perseverance', a year later they'd put the funding together to open a new home.

Today, it is clear a slow recovery is underway in Sabine. She has her own room, goes swimming and shopping with the carers from the home. She takes guided walks with the other residents to the local village. Her medication is half what it was in hospital. She can dress and clean herself and is moved, when helped to put on her make-up, to say, smiling: 'I'm pretty!' But her terror at separation is heart-rending. When Sandrine bends at one point to help her unlock her chest, she loses herself in the smell of her sister's hair. 'Ah, Sandrine's hair... Sandrine's hair...'

When the film was finished, Sandrine arranged a screening for Sabine and the residents of the home. It was, she laughs, 'very funny; very noisy. Sabine's got a DVD now, and for a while she watched it every day. But it got too much, so the doctor said, "We are going to watch it once a week now." It was getting boring for the other residents.'

For her mother and siblings, though, the film raised more complex feelings. 'Some of them were against the idea of the film to begin with, although some of my sisters were happy when they saw it. But it was disturbing for all of them, because they'd forgotten what Sabine was like before, so when they saw the juxtaposition, it was terrible.' For her mother, who no longer sees Sabine, the biggest shock was seeing her as she is now. 'For her,' Sandrine says, 'Sabine is the Sabine before.'

· My Name is Sabine opens at the ICA in London (020 7930 3647; www.ica.org.uk) on 20 June, and then at selected cinemas nationwide. The ICA screening is at 6.15pm, after which there will be a question-and-answer session with Sandrine Bonnaire.