Possibly Wes Craven's made-for-TV STRANGER IN OUR HOUSE ('78):
http://www.imdb.com/title/tt0078330/?ref_=fn_al_tt_1It's also known as SUMMER OF FEAR. From the IMDB summary: "A country family of five takes in cousin Julia, whose parents recently perished in a car crash. Julia extends her influence over the family and those around them, fooling all except for Rachel, the teenaged girl who knows her cousin is up to no good."