Would this violation make the pawn invalid without putting my brother in danger of forgery prosecution?
if it makes the continued pawn contracts invalid, you should simply plan on giving the car to the pawn shop since you have not paid your original pawn loan and I am sure they would consider it well beyond any grace period and would now claim title to your vehicle.

If you turn your brother in for forgery, be sure you are willing to send him to jail. If he argues you said he could sign in your name, while it is technically forgery since he signed your name, he can argue you gave him the right to represent you but only errantly and ignorantly signed your name rather than his acting as your agent.