Ringer (2011)

Six months after deciding to pull herself together and get sober, Bridget witnesses a murder. Realizing her life is in danger and not trusting the FBI to protect her, she runs to her wealthy twin sister, Siobhan, in New York. Things start out well—with the two patching up a fractured relationship—when Siobhan suddenly goes missing. Bridget assumes her sister's identity, but soon learns that Siobhan's life is not as perfect as it looks and she is no safer than she was before.