I understand your reluctance to incur expenses which, by the way, can be an expense of the trust and therefore paid from the principal. However, I think it would be
a small amount of money well spent for you to sit down with a lawyer and precisely determine what protocols you need to follow in accordance with the terms of the Will that established the trust to comply with the law and the terms of the trust. That being said, an IOU is just another phrase for a promissory note which is a written evidence of a debt. However, even if there is no IOU, there may still be a debt. So long as the money is owed legitimately, regardless of the form of the IOU, it is probably an enforceable debt of your brothers. I urge you to contact a lawyer to evaluate each of the debts and to guide you as to future management of the funds.