An Ember of Darkness by Faith Sloan:
 
Abandoned with a reclusive Coven, Ember is an orphaned witch who finds refuge only with her adoptive brother Rhydian. But when betrayal and exile rip them apart, she must navigate a treacherous new realm alone. There she draws the attention of the vampire Prince Aldric, who captures her and offers her protection. The catch? She has to become his Chosen.

As Ember struggles to understand Aldric’s heart and resist the dangers of his world, Rhydian desperately seeks to reunite with her.

But the ruthless King plots to exploit Ember’s powers for domination. Should Rhydian fail to find his sister in time, the vampires’ apocalyptic ambitions may be realized at last.

To save Ember and their world from endless night, Rhydian must rely on wits and newfound friends before the King’s plan comes to fruition. For in his hands, Ember’s magic could bring civilization to its knees.