La cravache (1971)

The unfortunate boy in this French movie hasn't got a reliable friend, and he needs one. His father, perhaps unhinged by the death of his wife, beats and torments him almost constantly. His older brother, instead of protecting him, keeps watch over him for the father. His best friend, a homosexual, is in love with him, but the boy is in love with the best friend's sister. When the best friend catches the two in bed together, all hell breaks loose.