It has been reported in the hadith: “No legacy is to be left to an heir unless the other heirs are in agreement.” And if the other heirs don’t agree to this, then the inheritance is split amongst the children, the male receiving twice the share of a female, unless there is another heir.
Dr Yusuf qaradawi says on rulings on wills for the heirs:
Disobeying parents in general, especially the mother, is from one of the biggest sins, after associating partners with Allah SWA, but it is impermissible for parents to forbid a disobedient child from receiving his right in inheritance, for Allah SWA has said in the verse concerning inheritance of children: “Allah commands you for your children’s (inheritance)” and He said at the end of the verse: “Your parents or your children – you know not which of them are nearest to you in benefit. [These shares are] an obligation [imposed] by Allah. Indeed, Allah is ever Knowing and Wise.” Al Nisa: 11
And the Islamic law doesn’t forbid inheritance from anyone, apart from one who kills an heir, there is no inheritance for a killer.
An Islamic Will is tied by two restraints:
1. The will is only for a third of the inheritance “And a third is a lot” as it came in the sahih hadith:(agreed upon by Sa’d bin Abi Waqas – Al lu’ lu’ wa Al Marjan 1053) Ibn Abas RA says: I recommend that people reduce the proportion of what they bequest by will to a fourth (of the whole legacy), for Allah’s Messenger (ﷺ) said, “One-third, yet even one third is too much.” (Agreed upon as it came in Al Lu’lu’ wa Al Marjaan 1054) the meaning behind this is that the prophet ﷺ wished that people would decrease the inheritance from a third to a fourth.
2. That the will is not for the heir, according to the hadith: “There is no will for the heir”- narrated by Al daarqatni of Jaber and it is in the small compiled sahih 7441)
This is because the will for the heir is forbidden as according to the scholars, only if the other heirs agree upon it, because the prohibition is from their right and if they allow a will to be written, they have forfeited their right. If they do not allow it, then it is not allowed for it to be implemented, because it is done on something against what the prophet ﷺ ordered, and so it reflects upon the one who does it.
If it is implemented in a resourceful way- like buying and selling of the inheritance, or through man made laws then the sin is on the one doing the will, and the one who is receiving the will as well, as they are trespassing the limits of Allah SWA.