The Cousins
Image can be found at https://www.piqsels.com/en/public-domain-photo-zbino
The Cousins by Karen M. McManus
Text from the back cover:
Milly, Aubrey, and Jonah Story are cousins, but they barely know each other, and they've never even met their grandmother. Rich and reclusive, she disinherited their parents before they were born. So when each cousin receives a letter inviting them to work at her island resort for the summer, they're curious. The parents are clear on one point -- not going is not an option. But when the cousins arrive on the island, it's immediately clear that their grandmother has different plans for them. And the longer they stay, the more they realize how mysterious -- and dark -- is their family's past. The entire Story family has secrets, and whatever pulled them apart years ago isn't over. This summer, the cousins will learn everything.