I suspect you are wanting to do a prepaid funeral plan which is extremely common these days.
If your brother is mentally incapacitated is there a power of attorney? Or is the Public Guardianship office acting on his behalf? If the former, the attorney can withdraw the money on production of the PoA to the bank. if the latter, inform the guardian what you are trying to do. However, what I say next may make this a waste of time.
If there are no parents who survive the deceased, the money will go to the brothers and sisters of the whole bloody first. HOWEVER, the funeral account has to be settled before the brothers and sisters get any of the money and they only get what is left after all debts and the funeral account has been settled.