Silent Fall
(1994)

Sylvie (Liv Tyler) is the killer.  She killed her father because he was sexually molesting both her and her autistic brother Tim (Ben Faulkner), and also her mother, because she knew about this and kept silent for the sake of her marriage.  Dr. Rainer (Richard Dreyfuss) figures it out shortly after he rejects a sexual advance from her, and then she drugs him and tries to have him drowned; however, Tim rescues him.
 
In the end, Dr. Rainer and his wife, Karen (Linda Hamilton), are shown as caretakers of Tim, who, at the same time, shows signs of noticeable improvement from his condition.

 Thanks Dr Mat!