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 is His slave and Messenger.
The will of your mother to divide her inheritance equally between her children males and females should not be taken into account because this contradicts the Ruling of Allaah. Allaah Says (what means): {Allaah instructs you concerning your children [i.e. their portions of inheritance]: for the male, what is equal to the share of two females.}[Quran 4:11]. Also, it is confirmed that the Prophet said: "Allaah has given each one his right and there is no will for an heir." [Ahmad]. Therefore, it is an obligation to divide the inheritance according to the Book of Allaah and the Sunnah of the Prophet . After this, if one of the heirs wants to renounce his/her right in the inheritance or part of it, he/she may do so provided he/she is an adult person who has a sound reasoning.
As regards the children of your brother, they are entitled to inherit the share of their deceased father in the inheritance of his father, but they have no right in the inheritance of their grandmother, because the grandchildren of the son cannot inherit while the son exists. Their father does not have any right in the inheritance of his mother because he died before her. It is the one who dies last who inherits the one who dies first and not the contrary. For more benefit, please refer to Fatwa 90126.
Finally, we advise you to take the matter to an Islamic court as the matter of inheritance is very complex because there might be other rights like debts and so forth, and these come in priority over the rights of the heirs. Therefore, the inheritance should not be divided before taking he matter to an Islamic court in order to preserve the rights of the live and the rights of the dead.
Allaah Knows best.