The Devil's Pawn — Marilyn Levinson

After fifteen-year-old Simon Porte's family is killed in an automobile crash, his father's brother, whom he's never met, brings Simon to live with him and his wife in upstate New York. Simon doesn't trust Uncle Raymond, and for good reason. Raymond is dying and using his diabolical powers to take over Simon's body. Simon must develop his own supernatural defenses to survive. With his dotty great-aunt, his young sister, and a pair of odd twins, he wages war against the evil Raymond and his cronies.