A co-signer takes on the full legal obligations of the loan, along with the primary borrower. If the primary borrower does not make the required loan payments, the lender will ask the co-signer to make them. The co-signer can be held accountable for the entire amount of the loan under the terms of the contract, in the same way as the primary borrower.
Unfortunately, you co-signed for someone who was a bad risk and now your responsible. You may have an small claims action against your brother for the money you are out but if he doesn't have anything then it may be tough collecting.