Is it permissible for a grandmother to make a will in favour of her grandchildren when her sons are still alive?
Question

                I have a question regarding specific rules of inheritance.  

1-My mother is divorced and has remarried.  The money that she will leave on to her relatives she obtained as a result of her first marriage, not her second.  

2- She has a son (me, her only living child from her first marriage), a husband, two grandchildren, and several brothers and sisters, one of whom lives in the same country (USA) as her.  

3- From my understanding of the rules of inheritance, she has the right to distribute up to 1/3 (or less) in any Islamically acceptable way that she wishes.  

4- After the first 1/3 (or less) is distributed, the remainder according to my knowledge is to be distributed as 1/2 to the son, and 1/4 to her current husband. Is this correct?  

5- Are there any other relatives who have rights of inheritance upon her?  

6- She wanted to simply divide her assets as 1/2 to me and 1/2 to her grandchildren, but I told her I wanted to make sure she did what was right in Islam, so that is why I am asking.
Praise be to Allah. 
          This question covers a number of topics: 
 
          1 – Those who are entitled to inheritance in this case 
          are: 
The father and mother, 
          and the husband and wife, and the sons, if there are sons, are not prevented 
          from inheriting. But each of them should take the share which is decreed 
          by Allaah.  But in the case of daughters, if there are also sons, 
          they inherit by right of descent [bi’l-ta’seeb] (The 
          word ‘by right of descent’ can be used with the sons and daughters but 
          notwith the brothers, sisters, uncles and cousins. 
          Therefore, I would think thatthe word ‘by agnation’ 
          is more suitable because it means the blood relationshipfrom the side of the father )  on 
          the basis that each male gets the equivalent of the share of two females. 
 
          In the case you mention, those who are entitled to inheritance 
          are: 
 
          The husband, who is entitled to one-quarter because there 
          is a son (a descendent who is entitled to inherit)
 
          The son, who is entitled to the remainder of the inheritance, 
          because he inherits by right of descent [ bil-ta’seeb ]
Brothers and sisters 
          are prevented from inheriting by the presence of the son, so they have 
          no right to the inheritance. 
 
          2 – Ruling on making a will in favour of an heir: 
 
          It is not permissible to make a will in favour of any 
          of the heirs (i.e., those who are entitled to inherit according to sharee’ah). 
          It was narrated in a saheeh report that the Prophet 
          
          (peace and blessings of Allaah be upon him) said: “Allaah has given 
          to each one who is entitled his rights, and there is no will in favour 
          of one who is an heir…” (Narrated by al-Tirmidhi, 2047; classed as saheeh 
          by al-Albaani in Saheeh Abi Dawood, no. 1722.) Shaykh Taqiy al-Deen 
          said, The ummah is unanimously agreed upon this. 
 
          Unless that will is approved by the heirs (who are entitled 
          to inherit according to sharee’ah), in which case it becomes valid; 
          it is subject to their consent. 
 
          3 – When the will is to be considered. 
 
          The will is to be considered immediately following death. 
           
 
          Al-Muwaffaq said: we do not know of any dispute among 
          the scholars concerning the fact that the will is to be considered immediately 
          following death. 
 
          See al-Mulakhkhas al-Fiqhi by al-Fawzaan, part 
          2, p. 174. 
 
          Ibn Hajar said: The (scholars) are agreed that the one 
          in whose favour the will is made is to be considered as an heir (i.e., 
          one who is entitled to inherit according to sharee’ah) on the day of 
          death. Even if one made a will in favour of a brother who is an heir 
          (i.e., one who is entitled to inherit according to sharee’ah), as he 
          does not have a son to prevent his brother from inheriting, but just 
          before he dies a son is born to him, and that son prevents his brother 
          from inheriting, then this will in favour of the brother is valid. If 
          he made a will in favour of his brother at the time when he had a son, 
          and the son died before his father, then the brother becomes entitled 
          to inherit and thus the will becomes invalid. 
 
          On this basis it is permissible for your mother to make 
          a will in favour of her two grandchildren (your two children) so long 
          as they do not have any rights of inheritance. She may also include 
          her brothers and sisters in her will, on the condition that she does 
          not give more than one-third to them, because the Prophet 
          
          (peace and blessings of Allaah be upon him) said: “One-third, and one-third 
          is a lot.” (Narrated by al-Bukhaari, al-Wasaayaa, 2538) 
 
          With regard to giving something whilst one is still alive, 
          this is a gift. See the answer to Question No. 10447