What can I do about this aunt/niece situation?

Tell her that if she doesn't stop you'll Report her for shop lifting(go with her and secretly tape her with your phone or something of her do it) and if she doesn't stop take the tapes to the police. She most likely won't get arrested but will get a hefty fine. She should stop then cuz then she knows that if it keeps happening you'll keep getting her fined. It's wrong for her to do that and YOU need to stop her. It's your private business not her pieres. Hope I helped