What is the name of this book?

So I remember reading this book about a girl who was kidnapped during a field trip. She got lost and he took her because she thought he was a worker and thought he was helping. She was then forced to stay young and do things to him against her will. She then sees in the newspaper that her parents are having a new child and were still looking for her many many years later. When she is about 16 or so she is getting too big to sit in his lap and doesnt look as young as he likes. Saying that he wants a younger girl who he sends her out to find and lure into him. She finds a girl at a zoo with her brother. She flirts with the brother and convinces him to meet up with her at a park nearby with his sister too. All the while the man watching near by so she doesnt screw things up. She then tells him about being kidnapped and what she plans to do with his sister. So the boy and Annabell (the name the kidnapper names her) go to the park as they plan but he then brings a gun and kills the guy. they live happily ever after. But I cannot remember the name of the book. I remember almost the entire book because of my memory yet I cant remember the name of the book. I think I read it freshman year and i also thought it was a nominees book but i looked at them all and couldnt find it. I am now a senior so that was 4 years ago. If it helps any in finding the name of the book. Thanks in advance to whoever helps me remember the name. (: