In general, oral agreements are enforceable. There are exceptions, however. It depends on the specific terms of the agreement. In your case, it sounds like having an oral agreement that is enforceable is a good thing if, in fact, the agreement with your brother was that the debt would be obsolved on your borther's death. Contact an attorney familiar with Idaho contract law. They will be able to assist you in assessing your options. Do not delay as the passage of time will negatively affect your rights. I wish you the best.