Galaxy Girl — Karen Hutchins Pirnot

Allie Henderson is sure she sees movement in her treehouse. When she climbs the ladder to try to solve the mystery of the movement, she is startled to discover a mirror-image of herself. When the stranger meets Allie's family, it is Kasem, the aide of Allie's handicapped brother Peter who disovers that the stranger is really an accidental time traveler. Unfortunately, no one in this galaxy has any idea of how to get Allie's galactic cousin Eilla back to her own universe.