The Captive Kingdom (the Ascendance Series, Book 4)

Ascendant King Jaron believes that his kingdom, Carthya, is at peace, so he and his bethrothed, Imogen, are sailing home from a trade mission when their ship is attacked by Prozarians, and Jaron and several of his friends are taken prisoner; the Prozarian captain seems to believe he had something to do with his parents' deaths and they also know a great deal about Jaron's long-missing older brother, Darius, the rightful heir to Carthya--who may be alive after all.