Stepsister from Planet Weird

Fourteen-year-old Megan Larson is stunned to discover that her mother's exuberant new fiancé, Cosmo Cola, and his seemingly perfect daughter, Ariel, are actually aliens who've taken sanctuary on Earth. Neither girl wants to see their parents get remarried, so they join forces to split them up. But as their plot unfolds, Megan and Ariel become fast friends and begin to worry that their initial wish will come true.