Fat Girl (French: À ma sœur!, lit. 'To My Sister!') is a 2001 coming-of-age drama film written and directed by Catherine Breillat, and starring Anaïs Reboux and Roxane Mesquida. It was released in certain English-speaking countries under the alternative titles For My Sister and Story of a Whale. The film's plot follows two young sisters as they deal with coming-of-age, sibling rivalry, and desire while on vacation with their family.

Twelve-year-old Anaïs and her fifteen-year-old sister, Elena, are vacationing with their parents in the French seaside town of Les Mathes. Bored of staying in their vacation home, the two walk into town while discussing relationships and their virginities. Although the conventionally attractive Elena has been promiscuous, she is saving vaginal intercourse for someone who loves her, while overweight Anaïs would rather lose her virginity to a man she does not love.

They meet an Italian law student, Fernando, at a cafe. Later, Fernando sneaks into the girls' bedroom for a liaison with Elena. Anaïs is awake and watches their entire interaction. After a conversation about Fernando's previous relationships with other women, Elena consents to have sex with him but backs out at the last minute. Frustrated, Fernando pressures her through various means, including threatening to sleep with another woman just to relieve himself. Finally, Elena is coerced into anal sex as a "proof of love", although it is obviously a painful experience for her.

In the morning, Fernando asks for oral sex from Elena before he leaves, but Anaïs has had enough and tells them to let her sleep in peace. The next day, the girls and Fernando go to the beach. Anaïs sits in the ocean in her new dress and sings to herself while Elena and Fernando go off alone together. Later, as the girls are reminiscing together back at the house about their childhood, Elena reveals Fernando gave her an engagement ring while at the beach. Anaïs openly expresses her suspicions about Fernando's intentions. That night, Elena has vaginal sex, which she considers losing her virginity, with Fernando as Anaïs silently cries on the other side of the room.

Fernando's mother comes by the next day and asks for the return of the ring Fernando gave to Elena, as it belongs to her and is part of a collection of jewelry from past lovers that she keeps. On discovering Elena and Fernando's relationship, her mother angrily decides to drive back to their home in Paris. On the way back, she becomes tired and decides to sleep at a rest stop, where a man smashes the windshield of their car with an axe, kills Elena, and strangles their mother while ripping her clothes. When Anaïs exits the car and starts backing away, he takes Anaïs into the woods and rapes her. When the police arrive the next morning, Anaïs insists she was not raped.

Catherine Breillat's experience during principal photography inspired her 2002 film Sex Is Comedy, which revolves around shooting a sex scene from the film. Mesquida reprised the scene for the later film. Principal photography took place in Les Mathes, France from late 1999 to early 2000.

Breillat revealed she was concerned about censorship because Anaïs's breasts are displayed in the final scene of the film. "I actually wanted her not to have breasts, but her body changed between casting and the end of shooting. It's funny that, if she had been flat-chested, it wouldn't have been an issue", Breillat said.

The film received generally positive reviews from critics. On the review aggregator website Rotten Tomatoes, the film holds an approval rating of 74% based on 87 reviews, with an average rating of 6.5/10. The website's critics consensus reads, "The controversial Fat Girl is an unflinchingly harsh but powerful look at female adolescence." Metacritic, which uses a weighted average, assigned the film a score of 77 out of 100, based on 24 critics, indicating "generally favorable" reviews.