The Vanishing Game - Kate Kae Myers

Jocelyn's twin brother Jack was the only family she had growing up in a world of foster homes-and now he's dead, and she has nothing. Then she gets a cryptic letter from "Jason December"-the code name her brother used to use when they were children at Seale House, a terrifying foster home that they believed had dark powers. Only one other person knows about Jason December: Noah, Jocelyn's childhood crush and their only real friend among the troubled children at Seale House.