Impostors by Scott Westerfeld

Frey and Rafi are inseparable . . . but very few people have ever seen them together. This is because Frey is Rafi’s double, raised in the shadows of their rich father’s fortress. While Rafi has been taught to charm, Frey has been taught to kill. Frey only exists to protect her sister. There is no other part of her life. Frey has never been out in the world on her own – until her father sends her in Rafi’s place to act as collateral for a dangerous deal. Everyone thinks she’s her sister – but Col, the son of a rival leader, is starting to get close enough to tell the difference. As the stakes grow higher and higher, Frey must decide whether she can trust him – or anyone in her life.