The Young Queens by Kendare Blake

Three black witches, born to a descending queen. One would rise to become queen in her place. Perhaps the strongest of the three. Perhaps the cleverest. Or perhaps it would be the girl born under the best shield of luck.

Katharine, Arsinoe and Mirabella – three young queens born to fulfil their destiny – to fight to the death to win the crown. But before they were poisoner, elemental and naturalist, they were children, sisters and friends . . .