Can you hold out till end of the school year and then tell them that you would like the sister put into a summer camp group of some sort to get used to a group setting before starting preschool. That her behaviour at daycare is such that without this she won't survive in Sept. Ideally they do meaning you keep the boy and somebody else gets to tell the parents just how much of a "brat" the sister is not you. Either way it will do her good or she won't survive at preschool. It is very common around here for those in school to have a different caregiver than younger siblings.