Spider Baby or, The Maddest Story Ever Told (1967)

In a dilapidated rural mansion, the last generation of the degenerate, inbred Merrye family lives with the inherited curse of a disease that causes them to mentally regress from the age of 10 or so on as they physically develop. The family chauffeur looks out for them and covers up their indiscretions. Trouble comes when greedy distant relatives and their lawyer arrive to dispossess the family of its home.