All the Lovely Bad Ones by Mary Downing Hahn

Siblings Travis and Corey are troublemakers and everyone knows it. So they're half excited when instead of going to the dreadful summer camp their parents normally send them to, they are starting with their grandmother at her rural inn. Their excitement grows when they find out that the in is supposedly haunted and fake supernatural experiences to help their grandmother's business. What they don't realize is what - or who - exactly their actions will affect the most.