The Lying Game by Sara Shepard

The Lying Game is a drama television series set to air on ABC Family. The show The Lying Game is based on a series of novels of the same name by Sara Shepard. The network green-lighted the series in February 2011, with an August 15 premiere planned.
New York Times bestselling author Sara Shepard's new series The Lying Game, published December 7th 2010, weaves a mystery of long-lost twins, mistaken identities, a duplicitous circle of friends, and cold-blooded murder. Combining juicy plot lines, to-die-for lifestyles, a tangled web of friendship and romance, magnetic characters, and sharp-witted narration from the afterlife, The Lying Game is an alluring cross between Pretty Little Liars and The Lovely Bones.
Foster kid Emma Paxton has only just discovered Sutton Mercer, the wealthy twin sister she never knew she had, when Sutton drops off the face of the earth, asking Emma to cover for her at home, school, and with her friends—just for a few days. The story is about foster child Emma Paxton, who comes to believe she has a twin sister. She goes on Facebook, and sends a message to the girl, Sutton. Sutton writes back saying that they need to meet. However, Sutton is actually dead. Emma goes to the Sobino Canyon, Arizona. While waiting for her sister, she is attacked by a group of people. They take her across the street, and remove the burlap sack over her head. That's when Emma finds out that it's just Sutton's friends playing a prank.
The friends believe that Emma is Sutton, and take her to Nisha's party. Back at Sutton's home, Emma still goes on with the charade as being Sutton, since they are identical twins... (Summary by Wikipedia)