All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The mother`s waiving her share in her daughter`s inheritance is valid if she enjoys complete capacity. As for the father, it isn`t permissible for his children to waive his right to his share in his daughter`s inheritance. This is because the Wali`s (Guardian) acting on behalf of the incapacitated person is conditioned upon what is best for the latter and waiving financial rights doesn`t serve that end. The children of the man suffering from amnesia must go to an Islamic court to interdict their father and in order for one of them to be appointed as a trustee to be able to spend his money (Father`s) in what is best for him. And Allah The Almighty Knows Best.