I Left My Sneakers in Dimension X — Bruce Coville

KIDNAPPED TO DIMENSION X!
When Rod Allbright's cousin Elspeth arrived for a visit the day after school let out for the summer, Rod thought his vacation was ruined. But after they were kidnapped to Dimension X by a monster named Smorkus Flinders, Rod realized that Elspeth was the least of his problems. After all, what's a bratty cousin compared to cranky monsters, furious aliens, and the discovery that you are stuck in the middle of an interstellar power play?

And that doesn't begin to take into account the horrifying personal decision that Rod will be forced to make before his adventures are over.