A Woman's Torment (1977)

well-to-do couple, Don and Frances has been struggling in their marriage. Frances's mentally ill sister, Karen, who lives with them, is taking a toll on their romance and as such. They bicker about what to do with the grown woman - who's gone mute since staying with them - and has even turned to their psychologist chum Otis and his shrill, unsatisfied wife Estelle for advice. Upon hearing they have been considering having her committed to an asylum, Karen flees to an abandoned beach house on Fire Island, where she endures a series of gnarly sexual encounters that push the voices in her head to finally lash out through her murderous hand.