The Family Plot by Megan Collins
The Lighthouse family is "unnatural." That's what Dahlia's twin brother tells her before running away on his sixteenth birthday. It's not until ten years after Andy's disappearance that Dahlia returns to the family's isolated island mansion for their father's funeral and realizes that her brother never left the property at all. All this time, he's been buried in the family plot, in the grave set aside for their father, his skull split open with his own axe.
A serial killer has lurked on the island for decades, killing young women and branding them before leaving their bodies to be battered by the waves. Dahlia and her siblings were educated through the homeschool curriculum their mother devised, one that consisted of writing carefully researched reports about murder victims and holding annual ceremonies to honor their names. The siblings themselves were named after famous murder victims. Now, while Dahlia tries to figure out what happened to Andy, her older brother, Charlie, is planning to open the house as a memorial museum and her sister, Tate, is constructing an intricate diorama of the murder scene.
I really enjoyed this one. It's a mystery with a kind of traditional gothic feel to it. Even though I'm not a true crime expert by any stretch of the imagination, the author provides just enough context with her references that I never felt like I was missing too much.