Is it permissible to execute the mother's will to allocate one-third of the money to the youngest brother and spend the rest on the funeral and burial, or is all the money considered an inheritance from which nothing can be spent on the funeral?

A will bequeathing one-third to the youngest son is a bequest to an heir, which is Islamically prohibited and not enforceable without the consent of the other heirs. As for her will that money be spent from the estate for burial necessities, it is valid and should be spent in a reasonable manner without extravagance. The expenses for preparing the deceased are given precedence over the rights of the heirs. If the will regarding the preparation expenses involves extravagance, then only an amount that does not involve extravagance should be spent, and the excess should be returned to the estate.