A woman who had two daughters and one son died, knowing that the son died before his mother and left children. The deceased woman is indebted to one of her daughters. The mother stated in her will before her death that her debt should be repaid to her daughter. Moreover, she stated in her will that she vowed that one third of her wealth should be given to the children of her daughter after her death. My questions are:
Firstly: what is the Sharee'ah ruling on the debt of the deceased woman to her daughter that is mentioned in her will?
Secondly: what is the Sharee'ah ruling on the vow stated in the will of the mother concerning giving one third of her wealth to the children of her daughter?
Thirdly: do the children of the deceased son have a share in the inheritance of their deceased grandmother? What is the Sharee'ah ruling on this case? Please advise.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The debt should be repaid from the legacy of the deceased before it is distributed, then the money that is bequeathed to the children of her daughter should be given to them because they are not entitled to prescribed shares of the legacy. A will should not exceed the third of the legacy. The deceased woman's vow is considered a will because it was suspended on death; thus, it is deemed a bequest. Al-Kasani said in Badai’i As-Sanai’i: 

"What remains of the legacy after paying the debt and fulfilling the bequest should be distributed among the inheritors as follows; two thirds for the two daughters and the rest for the children of the son by agnation. It should be equally distributed among them if they are all males. If they include females, then a male should have double the share of a female."

We would like to draw the attention of the questioner to the fact that the issue of inheritance is serious and extremely sensitive. Consequently, it is not sufficient to depend on a Fatwa that was prepared by a person according to a question and an answer. Rather, it should be presented to Sharee'ah courts in order to be researched and investigated. There might be an heir who is only discovered after research. There might be other bequests, debts or rights that are not known by the heirs. It is known that these matters are given precedence over the right of the heirs in the wealth. Therefore, the legacy should not be distributed without referring to the Sharee'ah courts, if they exist, in order to achieve the interests of the living and the dead.

Allaah Knows best.