From Ear to Ear (1970)

How long does it take a razor sharp knife to slice... From Ear to Ear

Lucille, a paralyzed girl, is the scapegoat of her sister and her cousin Josine and Elisa. During a "game" that goes too far, they accidentally kill the lover of Elisa. Whilst attempting to make Lucille bear the guilt of the killing, they discover the frightful secret that would explain the vegetative state of Lucille.