Is it permissible to bequeath the entire estate of a deceased grandfather who has four daughters and a son to his grandson or granddaughter?

A grandfather's will for his entire estate is not valid. It is only valid for up to one-third of the estate unless approved by the adult, mature heirs. If the heirs do not approve it, the will is first executed from one-third of the estate. Then, the remainder is divided: the daughters receive two-thirds, as they are more than two, and the remaining portion goes to the brother by way of ta'sib (residuary heirship). It is important to note that matters of inheritance are complex and should be referred to the Sharia courts for investigation, as there may be hidden wills, debts, or additional heirs.