Seeds of Sin (1968)

Claris Manning is an alcoholic invalid who tyrannizes her children when her youngest daughter, Carol, invites her family to spend a Christmas week together. They include Matthew, a crackpot clergyman; Michael, a misogynist businessman whom is sexually attracted to Carol, but abusive to his wife Susan; Margaret, the eldest daughter, whom is currently dating a local tough named Jonathan; and Buster, the youngest son from a military school whom brings over his gay lover Drew. Meanwhile, the two live-in servants, Peter and Jessica, plot to kill Mrs. Manning to gain her money for themselves. But an unseen killer soon begins killing off all the characters present for their debauched lifestyles they have led to since leaving the household.