Which Way to Anywhere by Cressida Cowell

K2 O’Hero is a seemingly ordinary boy – after all, he and his twin sister Izzabird have been sworn to keep their family’s magical history a secret. Not even their infuriating stepsiblings, Theo and Mable, know that magic exists. They believe K2 to be the most hopeless person they have ever known. But K2 has a secret gift: he draws maps of worlds that are beyond the wildest of imaginations. Worlds with six hundred moons, burning rivers and dark, twisty jungles alive with plants that hunt by the smell of fear. But what K2 doesn’t know, is that the maps he draws are real. When their baby sister Annipeck is kidnapped, the warring stepsiblings will have to use K2’s gift to find a crossing point into one of those worlds and embark on a daring rescue mission.