I remember this horror movie from late 70s early 80s. I think it was based in the woods in a house close to a river or the river may actually run under part of the house.

Someone kills anyone that comes near the house.

I remember close to the end a teenager girl finds what I believe was a boy locked in a dark room with the mirrors covered.  She tries to help the boy by taking the cover off the mirror so he can see himself (I don’t think she even saw his face at this point).  When he turns to the mirror she screams and I think runs out but someone catches her and kills her (not sure if it was the deformed boy) I think the killer might have been a woman like maybe the mother.