The Loves of Cynthia (1972)
""Possessed by a satanical force" awakened her sister's abnormal desires- - to bait a trap of death!"
Presented by that King of Sleaze Harry Novak comes this film about a woman who leaves her film career in Rome(no longer wanted) and comes to stay with her sister who married a millionaire. While there she devises a plan for her brother-in-law's brother and servant to have sex with her sister after being drugged with a an aphrodisiac in the hopes that her brother-in-law will divorce her sister and marry her. Whew! Some plot. - via BaronBl00d at the IMDb