The Cursed Sea by Lauren DeStefano 

Wilhemina Heidle, the exiled princess of Northern Arrod, the girl thought dead by her family and friends, must return home. The only way to save Loom, the cursed prince of the Southern Isles, is to revisit the castle and discover the origins of her own curse.

But home is very different from how she left it—Wil’s unstable elder brother is now king, leading a war against the Southern Isles. And the rest of her family…. Read more.