The ones we burn / Rebecca Mix.

Enhanced descriptions from Syndetics:

An instant New York Times bestseller!

Love and duty collide in this richly imagined young adult debut about a witch whose dark powers put her at the center of a brewing war between the only family she's ever known and the enemy who makes her question everything.

Monster. Butcher. Bloodwinn.

Ranka is tired of death. All she wants is to be left alone, living out her days in Witchik's wild north with the coven that raised her, attempting to forget the horrors of her past. But when she is named Bloodwinn, the next treaty bride to the human kingdom of Isodal, her coven sends her south with a single directive: kill him. Easy enough, for a blood-witch whose magic compels her to kill.

Except the prince is gentle, kind, and terrified of her. He doesn't want to marry Ranka; he doesn't want to be king at all. And it's his sister--the wickedly smart, infuriatingly beautiful Princess Aramis--who seems to be the real threat.

But when witches start turning up dead, murdered by a mysterious, magical plague, Aramis makes Ranka an offer: help her develop a cure, and in return, she'll help Ranka learn to contain her deadly magic. As the coup draws nearer and the plague spreads, Ranka is forced to question everything she thought she knew about her power, her past, and who she's meant to fight for. Soon, she will have to decide between the coven that raised her and the princess who sees beyond the monster they shaped her to be.

But as the bodies pile up, a monster may be exactly what they need.