i would go about helping her find another afforadable housing situation.
explain to her clearly that it is more stressful than you thought it would be to have another family living with you at this time in your lives.
give her a date to be out by and help her obtain that goal.
most of all be nice about it and understanding. she probably really needs friends right now.
as far as her dd rubbing off on your son i wouldn’t worry about that too much. all kids have tantrums from time to time and would get the same exposure from kids at preschool, a local park or wherever. as long as you enforce your limits with him he should be ok.