The Loves of Cynthia

Via BaronBl00d at the IMDb: "...this film [is] 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."