Esprit de Famille (1948)

A blonde woman drops by her sister's apartment unexpectedly and discovers her brunette sister having sex. While hiding behind a curtain, the blonde sister is so aroused that she begins to masturbate and is discovered. Angry that their sexual liaison has been disturbed, the man grabs the blonde and proceeds to spank her and then whip her with his belt. When the brunette objects to her sister's treatment the blonde helps the man bind the brunette's wrists and string her up from the ceiling and then beaten. The blonde and the man proceed to have sex while the tied-up sister watches. Later the sisters engage in mild lesbian familiarity.