MWP, I guess it goes without saying that they are out of control if you say they are stealing from your mom like that, underhanded way of doing it though, while she is sleeping.
I still think she is the one who has to take action. You really can't because look at the attitude you got from Family Services and Juvenile Aid. Maybe she should go to Family Services for some guidance.
She has to lay charges, if only to get them straightened up, its called tough love.