Jean Rollin – Gamines en chaleur aka Monique (1979)
Monique is a horny but frustrated virgin teenager living with her aunt and uncle. The aunt is very puritanical or pretends to be. The maid, Yvette, sympathises with Monique and together they peep on the aunt and uncle having sex. Yvette breaks the deadlock by seducing the uncle and catching the aunt masturbating with a cucumber in the bathroom. Her boyfriend (“Dominique”) catches Yvette and the uncle in the act, and, as far as can be gathered from the events (I don’t follow the Italian-dubbed dialogue) blackmails him to agree to inviting his friends round for an orgy.