Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Waqf (Religious Endowment) is an act of obedience which draws a Muslim closer to Allah the Almighty and a continuous charity for which he/she is rewarded after his/her passing away. Scholars are agreed that the condition of the donor is considerable as long as it is lawful.
An-Nawawi {May Allah have mercy on him} says: "The condition of the donor as regards the percentage, the characteristics of the beneficiaries, and the time must be observed. Thus, if he constituted the waqf for his children and stipulated that they get equal or unequal amounts, then his/her condition must be fulfilled." {Rawdat At-Talibeen, vol.2/pp.291}.
Consequently, the condition that the daughters benefit from the waqf as long as they are alive is considerable from the perspective of Sharia. An-Nawawi {May Allah have mercy on him} said: "Children of one`s sons and daughters are among the beneficiaries of family waqf unless the donor stipulated restricting it to those affiliated to him (his sons and their sons)." {Menhaj At-Talibeen}. Thus, the children of the daughters are excluded because they are affiliated to their fathers; not the donor. Accordingly, the family waqf stated in the above question is permissible and the condition of the donor is considerable. And Allah the Almighty Knows Best.