Double Identity by Margaret Peterson Haddix
So my only protection is a kindergarten teacher and a ninety-eight-pound female minister....And they don't even believe I'm in danger. As Bethany approaches her thirteenth birthday, her parents act more oddly than usual. Her mother cries constantly, and her father barely lets Bethany out of his sight. Then one morning he hustles the entire family into the car, drives across several state lines -- and leaves Bethany with an aunt she never knew existed. Bethany has no idea what's going on. She's worried her mom and dad are running from some kind of trouble, but she can't find out because they won't tell her where they are going. Bethany's only clue is a few words she overheard her father tell her aunt: "She doesn't know anything about Elizabeth." But Aunt Myrlie won't tell Bethany who Elizabeth is, and she won't explain why people in her small town react to Bethany as if they've seen a ghost. The mystery intensifies when Bethany gets a package from her father containing four different birth certificates from four different states, with four different last names -- and thousands of dollars in cash. And when a strange man shows up asking questions, Bethany realizes the's not the only one who's desperate to unravel the secrets of her past.