All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his Family and Companions.
The inheritance system in Islamic Sharia is considered from the legislative miracles of the Noble Quran because a fortune is divided in a just manner which observes the rights of parents, spouses, sons, daughters, brothers, sisters and others from the Asabat (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) depending on the degree of the relationship in line with the economic system of the Islamic legislation.
Moreover, Islamic Sharia has opened the door for the owner of property to receive great rewards from Almighty Allah by allowing him/her to bequeath one-third only of his/her estate by donating for charitable causes to please Allah, or donating to friends. Bequeathing more than one-third is impermissible unless the other heirs approved it. This is in order to preserve their rights for it was reported in a sound Hadith that the Prophet (PBUH) said to Sad when the latter was bequeathing his property: “O Sad! Give 1/3 (in charity) and even 1/3 is too much. No doubt, it is better to leave your children rich than to leave them poor, reduced to begging from others. “ {Bukhari&Muslim}.
Accordingly, you must execute your mother`s will by giving one-third of her estate to the poor and needy who aren`t among her legal heirs since the Prophet (PBUH) said: “ There is no bequest to an heir “ {Darqotni&Baihaqi}.
Therefore, it is impermissible to give the poor heirs from the bequeathed third unless the other heirs approved it. However, if only some of the heirs approved, the bequest will be taken from the shares of those who approved. And Allah knows best.