You Can't Have My Planet: But Take My Brother, Please — James Mihaley

Thirteen-year-old Giles is the last person anyone would expect to save the planet. he's not as charming as his little sister, and not as brainy as his goody-goody older brother. But when Giles witnesses an alien realtor showing Earth to possible new tenants, he knows he'd better do something. With the help of an alien "attorney" and the maddest scientist in middle-grade fiction, Giles just might save humans from eviction from Earth. Let's hope so. The alternatives are...not so hospitable.