Harley think she may be adopted [didn't we just read that two books back?] and it would explain how her eye color differs from the rest of her family, and why her parents have lost her birth certificate and get all weird when she brings up her suspicions. But mostly Harley needs it to be true, to help her cope with the spiraling nightmare of her life which is causing her to lose touch with who she is. Instead, she tries alcohol, smoking, and drugs, climaxing is a last ditch effort to sort everything out which takes her to New York.
This is a pretty dark and melodramatic book. It's not bad. The writing is good, the characters are believable. I bought the motives and all, but the payoff at the end will disappoint as you just hoped for so much more, and the plotlines developed early just seem to die in the end. As far as "mystery of where I came from" stories, this is lightyears ahead of the Face on the Milk Carton.