Erotic Passion(1981)

Since her parents' death, 19-year old Della lives with her aunt Marsha and Andrew, her old husband. One morning Marsha has sex with Alex, her husband's doctor. Right afterwords, Della seduces him at the beach while Marsha watches them from her window. The two women have a fight which results to Della leaving the house. She finds refuge at Maria's (a friend of hers) place but as it appears she is missing and the house is inhabited by Aris and Samantha. Della makes a pass at Aris, managing to infuriate Samantha. In the meanwhile, the local authorities are investigating the murder of Aris' business partner, Steve Brynner. When all trace leads to Aris, Della suggests a swap: She'll travel to Italy to get fake passports for the couple as long as they accept to murder Marsha.