Double Identity by Margaret Peterson Haddix
Bethany's parents start acting strangely as she approaches her thirteenth birthday. Her mom cries all the time and then one day, they drive across the country and Bethany is left with her Aunt Myrlie. Bethany over hears Aunt Merlie talking about some one named Elizabeth, but Bethany doesn't know who she means. When Bethany goes to the town YMCA, a woman there acts as if she has seen a ghost. Bethany desperately wants to know what is going on. Who is Elizabeth and why did her parents leave her with an aunt she didn't know?