The Lying Game By Sara Shepherd
Emma has a hard life in foster care and a harassing foster brother who is pointing the finger of accusation at her. His ultimate act is to show his mother a video of Emma playing at some dangerous game where she is strangled, only it is not Emma. Emma sets out to find the origin of the video and the other girl. She ends up becoming the other girl as she tries to discover what has happened and who did it. Even if you were identical how hard would it be to pretend to be someone you never met.