Fat Girl

ma sur is a 2001 French film directed by Catherine Breillat. It stars Anas Reboux and Roxane Mesquida. It was released in some English speaking countries under the alternative titles For My Sister or Fat Girl.

Anas Anas Reboux and her older sister, Elena Roxane Mesquida are vacationing with their parents on the French seaside. Bored of staying in their vacation home, the two walk into town while discussing relationships and their virginity. Although the highly attractive Elena has been promiscuous, she is saving herself for someone who loves her, while heavyset, overweight Anas thinks it is better to lose ones virginity to a nobody just to get it over with.They meet an Italian law student, Fernando Libero De Rienzo, at a cafe. While Elena flirts with him, Anas orders a banana split. Later, Fernando sneaks into the girls bedroom for a liaison with Elena. Anas is awake and watches their entire interaction. After a conversation about Fernandos 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 some other woman just to alleviate himself. Finally, Elena is coerced into anal sex as a proof of love, although it is obviously a painful experience for her. ........

Source: Wikipedia