You have no legal obligation to report this crime and no moral obligation to enlist in the drug war. That the money is owed to an unappealing creditor is beside the point. There are, of course, limits to your obligation. In many a Victorian novel, the children struggle for years to meet the debts incurred by their late parents. Spirits are crushed.
Hair turns gray.
You have no such obligation to pay your brother's debts from your own pocket, but insofar as his estate can make these payments, it should - his bills, his money, his responsibility.