Its not your obligation to prove this. If the credit card companies want, they can open a probate and investigate for themselves. You are not responsible for your brother's debts, unless you co-signed for them.
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I agree with David. Most creditors must file a claim in a probate proceeding within two years of the debtor's death. If they don't they don't get paid. If there is no probate proceeding, they can't file a valid claim. In my career, I've only seen one case where a creditor opened an estate (it was a bankruptcy trustee who filed a mulit-million dollar claim just in case any assets were discovered). Even if an estate is opened, the creditors don't get paid if your brother had not assets. It is admirable that you are trying to let the creditors know, but I don't think you have to spend a lot of your own money on something that is not your obligation.