Their grandfather left a will giving them the share of their deceased father
9-6-2011 | IslamWeb
Question:
Assalaamu ‘Alaykum; Please calculate the inheritance according to the following information -Does the deceased have male relatives who are entitled to inherit: (A son) Number 1 (grandson (from the son)) Number 2 -Does the deceased have female relatives who are entitled to inherit : (A daughter) Number 1 (A wife) Number 1 - The will which the deceased left behind and that is related to his inheritance is : My grandfather (May Allah have mercy on his soul) left a will (Wasayah) stating that me and my brother will get the amount of share from my grandfathers property, what our Father (May Allah have mercy on his soul) would have got. My Father (May Allah have mercy on his soul) passed away before ny grandfather ((May Allah have mercy on his soul). Can you please with respect to this Wasayah how will be the property divided among the heirs, the Heirs are my grandfather's wife, daughter, a Son and we two brothers. Please advise.
Answer:
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.
First of all, we should say that the sons of the son [grandsons] have no share in the inheritance as they are prevented by the existence of the direct son of the deceased.
So, if it is confirmed that your grandfather had made a will stating that you and your brother would take the same share as the share of your father if he had been still alive, then this is a valid will and it is an effective one as it is a will for a non-heir.
However, since the will exceeds one-third, then it is only one third that should be implemented of it unless all the heirs agree to what exceeds one third, then in this case they are permitted to do so.
In this case, it is a will that equals the share of a given heir because it equals the share of the son. Ibn Qudaamah 
said: “If he (the testator) willed (for a non-heir) what equals the share of a given heir, then he is entitled to that share apart from the legal shares of the heirs. This is the view of the majority of the scholars and this is the view of Abu Haneefah and Ash-Shaafi’i 
as well.”
Therefore, if the heirs agree to the entire will, then the issue will be solved as follows: the deceased left behind a wife, a daughter and a son, and then the same share of the deceased son will be added to the legal shares of the heirs. So, after taking out the will, the wife gets one-eighth as her legal share due to the existence of the children of the deceased. The remainder will be divided amongst the (living) son and the daughter by Ta’seeb (by virtue of having a paternal relation with the deceased and not having an allotted share, so they get what is left after the allotted shares have been distributed); the male twice the share of the female as 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]
Therefore, the inheritance should be divided into 38 shares, the wife gets 3 shares, the living son gets 14 shares, the daughter gets 7 shares, while the two sons of the deceased son in favor of whom the will was made get 14 shares, exactly like the share of their father if he were still alive, which means that each one of them gets 7 shares.
In case the heirs did not agree to what exceeded a third of the will, then the inheritance should be divided into 36 shares, the two sons of the deceased son get two-thirds of it, which is 12 shares, i.e. 6 shares for each one of them, and the wife gets one-eighth of the remainder after deducting the one-third of the two sons of the son, which is 3 shares. The son gets 14 shares, and the daughter gets 7 shares.
Allaah Knows best.