Is the mother's division of the inheritance among her children – where the old house is for the four daughters and the new one is for the two sons – considered a will that must be legally enforced? And is it permissible for the brother who built a structure on a portion of the agricultural land that he cultivates to exclusively own this portion without the rest of the heirs?

If a person dies leaving two sons and four daughters, their estate is divided according to the principle of "to the male, the share of two females." Thus, the estate is divided into eight shares: each son receives two shares, and each daughter receives one share.

As for the mother's will that the old house be for the daughters and the new house for the sons after her death, this is a bequest to an heir and is not valid unless approved by the remaining adult, sane heirs. If the heirs do not consent, or if there is a minor or an insane person among them, the houses must be divided according to Islamic law.

Regarding the land that one of the heirs cultivated and built upon, it is part of the estate and must be divided among the heirs according to Islamic law. One heir building on it does not make it their sole property; rather, it remains the property of all heirs. In case of dispute, the matter should be referred to the Sharia (definition) court or to knowledgeable scholars for judgment.