The Deep & Dark Blue by Niki Smith
After a terrible political coup usurps their noble house, Hawke and Grayson flee to stay alive and assume new identities, Hanna and Grayce. Desperation and chance lead them to the Communion of Blue, an order of magical women who spin the threads of reality to their will.
As the twins learn more about the Communion, and themselves, they begin to hatch a plan to avenge their family and retake their royal home. While Hawke wants to return to his old life, Grayce struggles to keep the threads of her new life from unraveling, and realizes she wants to stay in the one place that will allow her to finally live as a girl.