Ghost Ship — Kim Wilkins

EVER SINCE EMPEROR Flood drowned their kingdom and overthrew their parents, Asa and Rollo have been hiding out in Two Hills Keep. Then a mysterious stranger tells them their baby sister wasn't killed along with their mother and father. Instead, she was kidnapped by Flood's half-sister and secreted away in a castle to the north. If Una is still alive, there's only one thing for Asa and Rollo to do. They must take Northseeker, an invisible ship built of mist and shadows, and cross the danger-filled sunken kingdom.