Scream, Pretty Peggy (1973)
One of those ABC Movies of the Week, co-starring Bette Davis as the mother of a Sculptor named Jeffrey who hires college girl Peggy for domestic work. Soon enough Peggy is told that Jeffrey's insane sister Jennifer is living in a tiny apartment above the garage...
I'm surprised this well made proto-slasher hardly gets mentioned. Solid, with a few creepy moments.