Good half gone : a novel / Tarryn Fisher.

"Iris Walsh saw her twin sister Piper get kidnapped--so why does no one believe her? Iris narrowly escaped her pretty, popular twin sisters fate as a teen--kidnapped and trafficked and long gone before the cops agreed to investigate. Months later, Piper's newborn son Callum was dropped on their estranged mother's doorstep in the dead of night, with a note in Piper's handwriting signed simply, Twin. As an adult, Iris wants one thing--proof. Because she knows exactly who took Piper all those years ago, and she has a pretty good idea of who Callum's father is. She just has to get close enough to prove it. And if the police won't help, she'll just have to do it her own way--by interning at the isolated Shoal Island Hospital for the criminally insane, where her target is kept under lock and key. Iris soon realizes that something sinister is bubbling beneath the surface of the Shoal, and that the patients aren't the only ones being observed"-- Provided by publisher.