The Language of Ghosts by Heather Fawcett

Forced into exile on an enchanted, moving island, ex-princess Noa Marchena has two missions: reclaim her family’s stolen throne and ensure that the dark powers her older brother, Julian, possesses don’t go to his head in the process. But between babysitting her annoying little sister, Mite, and keeping an eye on the cake-loving sea monster that guards the moving island, Noa has her hands full. When the siblings learn that their enemies are searching [...]