The Burning Sea — Paul Collins

An exciting fantasy-adventure series by Paul Collins and Sean McMullen.
There is no lower rank than cabin boy on the warship Invincible. But Dantar knows he is important, because anyone who threatens his life gets turned into a pile of ashes. His older sister Velza is a shapecasting warrior, in a world where only men fight. Until now. Together they must solve the mystery of broken magic and escape the dragon.