1978 film by Daniel Daert

Oliver has a wife and a mistress -- quelle surprise!! Maude, his wife, decides to let him have his fun as long as she can control them. She brings Florence, a very attractive 18 year old, to live with them and to seduce Oliver. The plan works, and Oliver soon gives up his mistress for the younger Florence. As the plot develops, Florence not only captivates and controls Oliver, but Maude as well.