All the Lovely Bad Ones by Mary Downing Hahn
When Travis and Corey go to Fox Hill Inn for the summer they cannot resist the urge to recreate the Ghost sightings for the tourists. They begin to do a little "haunting" for themselves, then before long their pranks have the tourist flocking to the old inn, causing business to boom.
But then Travis and Corey find out that they are not the only ghosts at the Fox Hill Inn, Grandmother is not amused and blames them for the pranks pulled by the actual ghosts.Only Travis and Corey have the power to lay to rest the ghosts that they have stirred up. They begin by trying to find out what the ghosts want.