The Sister in Law (1974)

Robert, a singer-songwriter who has been traveling the country, makes a mistake when he goes to visit his brother, Edward who is in drug smuggling, and gets involved with his bored sister-in-law, Joanna who is about to divorce Edward and spending some time with her parent-in-laws. Robert and his brother's mistress, Deborah, try to help him get away from working in smuggling. It may or may not be a mistake, which turns out deadly.