The Lying Game by Sara Shepard

When foster kid Emma Paxton discovers she has an identical twin named Sutton Mercer—a wealthy girl living a life of privilege—she’s shocked. But when Sutton suddenly disappears, Emma finds herself assuming her sister’s identity to find out what happened. As she steps into Sutton’s glamorous but dangerous world of lies, secrets, and betrayal, Emma realizes someone close knows the truth—and that the next target could be her.