As a kid I recall seeing this horror movie on tv that I vaguely remember. I really only remember the last scene in detail. Can any of you horror buffs help me identify this? I believe its from the 70s (possibly 60s?):

A couple and their little boy move into this house. The mother cares for this old woman who lives in the attic, who is never seen by anyone else. In the end, the family is leaving this house. The mother runs back inside to the old lady. The house/roof begins to crumble down on the little boy's face. He runs in, after his mother, only to discover that there really is no old woman at all, but that his psycho mother is sitting there all dressed up as one in a rocking chair.

Can anyone help me out?