Ringer (2011)

Bridget is six months sober and getting her life back on track when she becomes the sole witness to a professional hit. She flees to New York City where she reunites with her estranged twin Siobhan. Wealthy, pampered, and happily married, Siobhan lives a fairy-tale life. Just as the identical twins are mending their frayed relationship, Siobhan disappears overboard during their boat trip, and Bridget decides to take on Siobhan's identity. She discovers shocking secrets, and not just about her sister and her marriage, and realizes she's no safer as Siobhan than as herself.