The Mafu Cage (1978)

Set in Los Angeles, the daughters of a deceased primatologist live in a decaying mansion. While one (Lee Grant) has a career as an astronomer, her younger sister (Carol Kane) is mentally unbalanced and habitually abuses and kills primates. Based on the play TOI ET TES NUAGES by Eric Westphal (adapted for the screen by actor Don Chastain) and directed by Karen Arthur. After a non competitive slot at the Cannes film festival, this psychological horror movie did well in Europe but was released as an exploitation horror movie in the U.S. It's well made for sure but I found it extremely unpleasant to sit through in spite of strong performances by Grant and Kane as the sisters. Disturbing because Grant knows her sister is batshit crazy and a danger to herself and others yet she refuses to get her sister help, preferring to take care of her herself with disastrous results. Still, there's no denying Arthur's intense direction results in a harrowing slice of horror that's not easy to shake off. With James Olson and Will Geer.