And I Darken by Kiersten White

Lada is everything a princess should not be, and her younger brother Radu is far too fragile to be a prince. Both are abandoned by their father and taken in by the Ottoman Empire as leverage against their home country, and it is during their time as prisoners that they meet Mehmed, the first and only person to truly understand and love them. As the trio grow together and become entangled in the politics of the empire, Radu wonders if he has finally found his place in the world, and Lada questions her loyalty to her home.