If the Father is no longer alive, and the Mother includes the daughters in the will, does this stand halachically?
The question is if the mother owns the property at all — meaning, did the father, before he passed away, ensure that the wife will inherit his property according to Torah law? It is possible that a legal will made by the father, together with the agreement of children, will suffice for this matter.
Assuming that the mother does own the property, she has the right to include daughters in her will, but she should ensure that the will is made both legally binding, and binding according to halachah.
This can be done through consultation with a competent authority. If the will is not made binding according to halachah, some authorities uphold its validity, but others dispute it, so that the best path is certainly to make the will binding according to halachah.