The Red Twins by Tony Belmonte

The Torcama Dynasty has ruled the small island of Saara for generations. Saara, besieged by the monstrous veshuti, is the last holdout of civilization, and though the mighty walls of the island keep the vile sea serpents from conquering the island, there seems no hope for renewed contact with the mainland. When the King betroths his daughter Princess Aelia to Earl Pedras, expecting her to do no more than breed heirs, she cherishes every remaining moment of freedom. But when the mysterious disappearance of the veshuti alters her destiny, the fate of every last Saaran depends on Aelias courage.