A Reflection of Fear (1972) Sondra Locke

A young disturbed girl lives with her mother and grandmother. One day her estranged father returns home with a female companion he introduces as his fiancee. Soon the girl finds herself in the midst of strange goings-on, which evolve into a web of murder. Throughout all this, her father starts acting less and less fatherly which leads his horrified fiancee to believe that he wants to sleep with his own daughter.