The Cousins by Karen M. McManus

Milly, Aubrey, and Jonah Story are cousins, but they barely know each another, and they've never even met their grandmother. Rich and reclusive, she disinherited their parents before they were born. So when they each receive a letter inviting them to work at her island resort for the summer, they're surprised . . . and curious.

Their parents are all clear on one point--not going is not an option. This could be the opportunity to get back into Grandmother's good graces. But when the cousins arrive on the island, it's immediately clear that she has different plans for them. And the longer they stay, the more they realize how mysterious--and dark--their family's past is.