Three Dark Crowns by Kendare Blake
On the island of Fennbirn, every generation brings with it a set of female triplets, all equal heirs to the throne, and each with her own magical abilities. Mirabella can spark flames or conjure up storms. Katherine can ingest poison without fear of death. Arsinoe has control over all natural elements like plants and animals. The three sisters must fight to prove who is most worthy of being crowned queen, so who is the strongest and who will perish and at the hands of her own sister?