70s 'stephen king' style movie, mentally disabled man, big twist at en


Hoping someone can help me remember the name of a 70s (possibly early 80s) movie I remember seeing late one night on TV in the early 90s, so I can try and find it again. Story was about a female psychiatrist trying to rescue a mentally disabled young man from an abusive family (his mother and two sisters), and it had a fantastic twist right at the end. Definitely had some Stephen King vibes to it.

The Baby, thatís the one! Thanks, I wonít spoil the twist at the end (though guess revealing there is one already spoils it), it really took me by surprise at the time. Surprised this film was only a PG.