Dwarves in the Dark — Christine M. Morgan

Sam Broderick and Stone Hammerfine are two boys from two different worlds, but with the same problem. Big sisters. Sam is an American kid who likes macaroni and cheese, video games, toy dinosaurs, and television. Stone is a dwarf from Underearth, where nobody ever swims or sees the sun, and bugs are for dinner. They both hate it when their parents go away, and leave their big sisters in charge. Garnet Hammerfine and Katie Broderick are exactly the same - bossy and mean. Always telling their little brothers what to do and treating them like babies. Never letting them have any fun. Making sure they clean their rooms and eat their dinners and wash their faces. It's awful. When Stone gets tired of it and runs away into the Howling Caves, Garnet needs help finding him. She brings Sam, Katie, and their brother Kevin through a magical silver doorway that connects their two worlds. Sam decides to prove he's not a baby by going off to find Stone all by himself. But when giant spiders want to tie them up in webs, and robbers with axes chase them through the dark tunnels, Sam and Stone wonder if maybe this time, just once, they should have listened to their sisters after all.