Die teuflischen Schwestern (1977)

Edna keeps her younger sister Millie drugged and chained to her bed. The drugs have made Millie a nymphomaniac who's endless supply of men that Edna providers her cannot satisfy her and seeing her sister unsatisfied gets Edna off. But Edna's main plan from all of this is to inherit her deceased parents' fortune which is supposed to all go to Millie on her 21st birthday, so if she can prove Millie insane she will get the fortune. But when one of the men appears to satisfy Millie, Millie discovers the outside world and the two will try to stop Edna's evil plans.