Teenage sex is full of traps. Romantic desire has no protection. Boys do it because it's expected of them. Virgins do it carefully.

Catherine Breillat made Romance, one of the most explicit and revealing French movies of the last decade. A Ma Soeur! deals with a more sensitive subject. It is not the act so much as the emotion.

Elena (Roxane Mesquida) and Anais (Anais Reboux) are sisters on holiday with their parents at a seaside resort. Family members appear oddly dysfunctional, as if communication has become a charade.

Elena is 15, a flirt and knowingly sexy. Anais is three years younger, fat and watchful. Fernando (Libero de Rienzo) is an Italian boy, who picks up on Elena.

"You'll always remember me," he says. "The first lover."

"Will you remember me?" Elena asks, plaintively.

Anais eats and swims. Elena is caught between her imagined passion for Fernando and the fear of losing her virginity. She can only do it, she tells herself, if she loves him enough to want to spend the rest of her life with him. It's nonsense and she knows it, but won't admit as much, because that would make her feel cheap. Anais knows it, too, and says so. The girls fight and argue and fall about giggling.

Fernando waits, like a shark in shallows. Anais waits, too, in tears. The Beautiful Moment is painful and short. On the way out, he says, "Next time I'll teach you about love."

As a rites-of-passage, it is Anais who walks away the wiser. She sees clearly. Her bingeing is compensation for inflicted loneliness.

Breillat tags on a shocking ending that upsets the rhythm of the piece. It cannot be criticised as trivial and yet feels part of another film. Reboux and Mesquida bring a natural intelligence to demanding roles. Their contribution lends resonance to an otherwise difficult subject.