The Roanoke Girls

This isn't a thriller, and barely a mystery. It is a family drama. You know the big secret of the family less than halfway in, and the culprit of the main mystery considering the number of characters is more than obvious from the start.

also cw for incest and pedophilia. besides the obvious the protags main love interest as a teen is also much older than her and this is never called attention to even when they reunite when older. Found that strange in a book with such themes. Almost as it's going "hey this dude might have been taken advantage of a teen but at least they're not blood related!"