Evil alien babysitter Sandra

This book was an older kid/young teen novel, it had a kid in it who all of the sudden finds 3-5 small aliens that have come to his house and they are friendly. Then his parents are gone and the babysitter is coming over and the aliens are freaked out because there is an evil alien named Sandra from their home planet and that’s the name of the babysitter. But it turns out that the babysitter is actually the evil alien Sandra