Can you give me a detailed summary of New World by Gillian Cross?

"New World" by Gillian Cross tells the story of teenagers Nick and his younger sister Lily who discover a hidden world where they must confront challenging situations and make difficult choices to survive. They encounter strange creatures and meet other characters who help or hinder them along the way. As they navigate this new world, they realize the importance of family, loyalty, and bravery in the face of danger.