for All the Lovely Bad Ones by Mary Downing Hahn
Travis and his sister, Corey, can't resist a good trick. When they learn that their grandmother's quiet Vermont inn, where they're spending the summer, has a history of ghost sightings, they decide to do a little "haunting" of their own. Before long, their supernatural pranks have tourists flocking to the inn, and business booms.
But Travis and Corey soon find out that they aren't the only ghosts at Fox Hill Inn. Their thoughtless games have awakened something dangerous, something that should have stayed asleep. Can these siblings lay to rest the ghosts they've stirred?