Psychological Thriller. Book about siblings that were adopted (I think). I remember a black and green cover with possibly a tree on it. Spoilers ahead.

[Lauren]

I read this book between 2009-2011. I don't remember much, but I remember that it mainly focused on the girl and I think her brother had been adopted out to a different family or maybe he ran away; her and her brother would write notes to each other and she's trying to find him; One instance she has a flashback of being in I think a truck with her brother and I think there's an accident, and he slams his head against the window. She remembers going to a hospital and being in the chapel. At some point in the book either she or the reader (or both) find out that he actually died at that point and she was sending herself the notes and going back and forth from her personality to his.

[Kris]

Lauren, how old are the siblings in the present day? Is this book for adults or teens? (If she's an adult, what's her job?)