Spider Baby, or The Maddest Story Ever Told

This highly influential cult classic from writer-director Jack Hill tells the demented and darkly comic tale of the Merrye children -- Elizabeth (Beverly Washburn), Virginia (Jill Banner) and Ralph (Sid Haig)-- all of whom suffer from a rare genetic malady that causes its victims to mentally regress to a condition of "pre-human savagery and cannibalism."
The children live in the old family mansion, and though under the cautious guardianship of chauffeur Bruno (Lon Chaney Jr.), they manage to terrorize anyone or anything which passes through the gates. So when the family is visited by a pair of distant relatives and their greedy lawyer, a wild night of murderous thrills ensues. (official distributor synopsis)