Complex sentence: His training had not been completed when he was captured by Viking raiders, Jack and his younger sister Lucy (ages 11 and 5 at the beginning of the book) are to be sold as slaves as soon as they reach land.
Split into simpler sentences: His training had not been completed when he was captured by Viking raiders along with his younger sister Lucy (ages 11 and 5 at the beginning of the book). The pair are to be sold as slaves as soon as they reach land.