What is the ruling on a brother who holds a general power of attorney from his deceased father, but has not obtained a death certificate for the father, claims that the father had no money, and refuses to give the rest of the heirs (children of the deceased second wife) their share of the inheritance?

As soon as the father dies, his power of attorney to your brother becomes null and void, and the brother is not permitted to act on what he was authorized to do. Whatever the agent did after the death of the principal is invalid, because agency is revoked by the death of either party. Whatever the father left behind as inheritance is a right for all his heirs according to their legally prescribed shares, after the settlement of debts and bequests, if any. If the brother took anything from his father's money unjustly, he is an oppressor and consumes wealth unlawfully, and the matter can be brought to court for a ruling.