The Face on the Milk Carton by Caroline Cooney

This book is the first in the series; I actually read the third book first! Janie is a typical sophomore in high school and sits down to have lunch with her friends. She picks up her friend's carton of milk carton and notices the face of a three-year-old girl on the side. She freezes and thinks, "I remember that dress." Is it possible that she could have been kidnapped? Janie becomes obsessed with finding out about this little girl, and memories start to pop back into her head. Smiling faces, ice cream, twin brothers, and a sister. Janie has two loving parents, but they couldn't have stolen her from another family, could they? With the help of her new boyfriend, Janie sets out to discover the truth. A missing birth certificate, a sister her parents won't talk about, and all of those memories. What IS the truth?

You really need to read the books in order, because the next one picks up where the previous book leaves off. The conflict is pretty huge, as Janie is forced to question her whole life. She loves her parents, but has it all been a lie? She doesn't want to believe she actually belongs to another family, but what if it's true? Don't they deserve to know their long-lost daughter is alive and well? As I mentioned, the series will read like one long book, so be prepared to get hooked on the adventure!