Maybe shes bored and trying to keep busy while trying to help you. Have you tried to ask her if she would mind getting a meal ready, making cookies, doing the laundry or maybe try to get her involved in quilting - ask if she wouldn't mind cutting a quilt up for you for starters(show her how if she doesn't know how). Explain how not to throw away scraps as those can even go into a quilt. Maybe find some scrappy quilts on here to show her and some that even use the edge. When you're at somebody's house babysitting you can get bored very easily with nothing to do. Give her something to do and/or talk to her about bringing something she enjoys doing(if she doesn't want to learn quilting). Tell her you know she means well and you appreciate having a trust worthy person and someone who loves your child(ren)watching over them but anything in your house could have a purpose and you would appreciate it if she would ask you before tossing or putting anything away unless she or the kids get it out while she is there.
Good luck and hope everything works out.