The Cousins by Karen M. McManus
The four children are suddenly dropped by their mother with a single sentence:You know what you did. They never hear from her again. Years later, when 18-year-old cousins Aubrey, Milly and Jonah Story receive a mysterious invitation to spend the summer at their grandmother's resort, they have no choice but to follow their curiosity and meet the woman who's been such an enigma their entire lives.
This entire family is built on secrets, right? It's the Story legacy. This summer, the teenagers are determined to discover the truth at the heart of their family. But some secrets are better left alone.