Trying to find title/author of a book I read in 1986ish. The plot involves a brother and sister going to a house and meeting a girl (maybe their new stepsister) who seems to know about the occult. Poltergeist hoax ensues, stepsister is guilty.
Possibly The Headless Cupid by Zilpha Keatley Snyder. It was published in 1971 -- a Newberry Honor book by a young adult author whose books have been popular since the 1960's. The plot does revolve around a poltergeist hoax and the difficulties of a new blended family.
A professor with four children marries a woman with a 12-year old daughter, Amanda. They move to an old house that was once the scene of poltergeist activity. Amanda is obsessed with the occult and offers to teach her new siblings witchcraft. But her lessons seem to be designed to cause trouble for her new step-brothers and sisters, and to exact revenge on her mother for remarrying. After holding a séance, Amanda seemingly resurrects the poltergeist. Stones are thrown about the house and items are broken. But the oldest boy, David, comes to believe that Amanda is faking the poltergeist attacks and catches her in the act.