Satan Was a Lady (1975)

Large-breasted Terry has an affair with Victor, the fiancé of her sister Claudia, Victor. She also has an affair with another man, Bobby. Victor isn't quite the "wronged" fiancé, however; it turns out that he's having an affair with a hot young thing. In addition, Adaa--Terry's mother and Claudia's stepmother, has a plan to scare terrorize Claudia and scare her out of her inheritance.