Never Have I Ever by Sara Shepard

Never Have I Ever, published August 2, 2011, is the second book in the four book thriller series The Lying Game by Sara Shepard, about twin sisters separated at birth.
In this second installment of Sara Shepard's "The Lying Game" series, Emma Paxton finds herself delving deeper into the mysterious life of her- now dead- twin sister, Sutton Mercer. The unknown murderer seems to be watching her every step and she must carefully maneuver maintaining a low-key investigation into her sister's death whilst not alerting the killer to her intentions.
Emma continues to pose as her twin sister, Sutton. She continues to find out clues or anything to find out who has killed Sutton. Between finding clues and who the actual killer is, she continues to play, The Lying Game. The series follows Emma, a foster kid who can't catch a break who finds out she has an identical twin sister, Sutton, who unlike Emma was adopted by wealthy parents and is seemingly living an ideal life. After their initial meeting, Sutton talks Emma into stepping into her life for a few days while she pursues a lead on their birth mother. Initially excited to do this favor for her sister, Emma soon learns that Sutton has gone missing and could be in trouble. Now, Emma must decide whether to come clean to Sutton's family and risk her own safety in the hope of uncovering her twin sister's true whereabouts, along with the truth about why they were separated in the first place... (Summary by Wikipedia)