My name was Eden : a novel / Eleanor Barker-White.

When her daughter Eden came home from the hospital, Lucy was profoundly relieved. Eden had survived a drowning incident and had no apparent brain damage, no serious injuries, not even a scratch on her. Lucy fervently welcomed having a second chance at being the good mother she should have been before her teenager's accident. Until Eden tells her that Eden isn't her name. Until she starts calling herself Eli. The name Lucy had reserved for Eden's unborn twin. Don't worry, says the doctor. Eden is completely fine, says her husband. Of course I'm fine, Eden says, with that strange new smile of hers. I didn't die. I'm here. But Lucy knows something's very wrong with Eden. She's not her maddening, complicated teenage girl anymore--this straight-backed, even tempered, steady-eyed child in her house is someone else entirely. Eden, it seems, is the twin who disappeared...