Casters and Crowns by Elizabeth Lowham

When Crown Princess Aria sees her kingdom on the brink of war with the magic-wielding Casters, she takes it upon herself to speak with Widow Morton, the most dangerous Caster in the realm. If successful, Aria will not only ensure peace but also prove to her father that she is a worthy heir. But when her negotiations fail, Aria is dealt a terrible curse: Over the course of one hundred days, she will be unable to sleep, her body and mind doomed to waste away. And then the curse will pass to her sister, until every member of the king’s line is dead.
Aria’s best hope for breaking her curse is to seek help from another Caster—the handsome and charming Baron Reeves. He may be entitled to a place at the court, but as a magic user, can he truly be trusted? She vows to not be fooled by his dimpled smiles or his devotion to his brothers, but as she spends more time with him, she discovers that her heart has other ideas.
Baron has no desire to see the princess suffer—especially since she might be the key to changing the king’s opinion on Casters—but he has secrets to protect and ghosts of his own. The last person he tried to save using his magic died, and he’s not willing to risk harming someone he loves again.

As the curse continues its deadly countdown, both Aria and Baron must risk their lives and their hearts to save each other and unite a kingdom.