Galaxy Girl (Skymasters Book 1)
Preteen Allie Henderson goes to her treehouse to explore and comes face-to-face with a mirrow image of herself. It takes awhile to ascertain that the visitor named Eilla is really Allie's galactic cousin from a parallel universe. While the girls attempt to camouflage Eilla's read identity, a top team of scientists from MIT ponders just how to get Eilla back to her own universe.