What is the ruling on debt concerning a deceased brother and the questioner, especially after the questioner publicly absolved his deceased brother of his debt, and then discovered much larger debts for which there is no official proof except for "blank" trust receipts?

As for you, you are not obligated to pay your brother's debt as long as he did not leave an inheritance from which the debt could be paid. This is if you knew about the debt and acknowledged it; otherwise, his claim would not be accepted from anyone without legitimate proof. As for your brother, if he was in debt or held a trust, his conscience remains burdened by that, and he will be held accountable for it before Allah. This varies from person to person depending on the negligence in fulfilling rights and safeguarding trusts, and depending on the intention to pay and the resolve to do so when unable.