What is the theme in Shipwreck by Gordon Korman?

The value of family is the major theme in the book. Will and Lyssa Greenfield are a brother and sister who have yet to learn the value of family, and the importance of the bond that keeps a family together. Will has been jealous of his sister for some time. Part of that could be attributed to the fact that Will was the older child. He began life with his parents all to himself. Then Lyssa had come along, sweet and cuddly, requiring the undivided attention of his mother and Will discovered that he was resentful of her good looks, charm and the constant attention she got. As Lyssa got older, it only got worse.