Handling an Insolvent Estate

My wife's brother recently died. As nearest kin, she's handling his affairs and hopes to keep it as simple as possible. Mostly, his affairs are very simple: no spouse, no real estate, and no bank accounts. We have already cleaned out his apartment and directed that his mail be forwarded to us. He owned a new truck, on which more is still owing than we could sell it for. Also, it appears he had a few credit cards with balances owing. Are we responsible for these things? How should we proceed?