Assuming that your brother in law did not continue with the finance payments, the finance company would reposess the car � sell it and demand any outstanding amount of the loan from him.
He would have a very good case in law to sue you for any amount that he had to settle with the finance company.
Although he may not have a specific agreement with you that you pay this money in the event of you defaulting on the finance, a court would reasonably agree that you were liable, as result of your actions.