I have a 15 year old stepson. His aunt asked to borrow over $800 from him and specifically told him not to discuss it with us (his mother and I) or his father (whom she also owes money to). She refuses to deal with his mother for terms of paying it back and we set a deadline for her to return the money, however in the meantime, she has had a car repossessed and asked her own son to borrow money (which he wisely refused). What are my options?

Ok, lets get some terms straight. It is not theft to borrow money. So it falls under a different category. Unless the money was taken without permission. In the case of a debt it turns into a civil matter. Was anything put in writing? You could try your local small claims court and get a judgement against her and proceed with options from that point to collect.