Perhaps I see why she wants to get away from her siblings. Since you are the teacher you should be the one to set the rules and tone of the learning session. Saying 'no' is the beginning. If she will not use your equipment properly or listen to instruction you should say 'no'. If she has no allowance to buy her own twin needle, perhaps you can give her chores at your house to earn the money for one. She needs to know the cost. She also needs to know the consequences of not following instructions. This girl seems to need encouragement and you can be that person if you do not allow her to run wild in your sewing room.