There is consensus that the mother excludes from inheritance all kinds of grandmothers.
The Sunni legists state: In the absence of the mother, her mother represents her and inherits jointly with the father and the paternal grandfather, taking one-sixth in their presence. Similarly, there is no difference of opinion concerning the maternal and paternal grandmothers inheriting jointly. According to the Sunni schools, they are entitled to one-sixth, which they share equally among themselves.
The nearer grandmother excludes the more distant grandmother on her side. Hence the mother's mother excludes the latter's mother; the father's mother also excludes similarly. The nearer maternal grandmother (e.g. the mother's mother) prevents a remote paternal grandmother (e.g. the paternal grandfather's mother). The Sunni schools differ among themselves as to whether or not a nearer paternal grandmother, such as the father's mother, excludes a remote maternal grandmother, such as the maternal grandfather's mother (al-'Iqna' fi hall alfaz Abi Shuja', vol. 2, and al-Mughni, vol. 5, bab al-fara’id.). According to the Hanbalis, the father's mother inherits with her son; hence when they inherit together, she takes one-sixth and he the remainder.
The Imamiyyah say: If the maternal grandmother is present along with the paternal grandmother, the former takes one-third and the latter two-thirds, because the maternal grandfathers and grandmothers take one-third irrespective of whether they are one or more, dividing their share of the estate equally, and the paternal grandparents take two-thirds, whether one or more, and divide their share with disparity (a male taking twice a female's share).
The four Sunni schools concur that the father's father represents the mother in her absence and inherits jointly with the son, like the father, though differing from the father in respect of his wife, the father's mother, because she does not inherit with the father, except in the opinion of the Hanbalis, but inherits jointly with the paternal grandfather, i.e. with her husband.
The father also differs from the paternal grandfather in the case of both parents jointly inheriting with a spouse; here, the mother inheriting jointly with the father and spouse receives one-third of the remainder after deducting the share of the spouse, and while inheriting jointly with the paternal grandfather and spouse she receives one-third of the original estate and not one-third of the remainder.
The four schools also concur that the paternal grandfather excludes from inheritance uterine brothers and sisters as well as the children of full and agnate brothers. These schools differ among themselves concerning whether the paternal grandfather excludes full and agnate brothers and sisters or if he inherits jointly with them.
Abu Hanifa observes: The paternal grandfather excludes all kinds of brothers and sisters from inheritance, exactly in the manner that they are excluded by the father. This is despite the fact that according to the Sunni schools the maternal grandfather excludes none of the different kinds of brothers and sisters, because he is included, as mentioned earlier, among distant kindred.
The Maliki, the Shafi’i, and the Hanbali schools and the two disciples of Abu Hanifa, Abu Yusuf and Muhammad ibn al-Hasan, state: Full and agnate brothers and sisters inherit jointly with the paternal grandfather. The manner of their inheriting with him is that he will be given the greater of these two: one-third of the whole estate or a brother's share. Accordingly, if there exist a brother and a sister, he will receive equal to a brother's share and take two-fifths of the estate, and if there exist three brothers, he will take one-third because a brother's share will be one-fourth. (al-Mughni, vol. 6, p. 218)
The Imamiyyah observe: The grandparents, brothers and sisters inherit together and belong to the same category. Hence if they exist together and are related to the decedent from the father's side, the grandfather and the grandmother will take the share of a brother and a sister respectively and the estate will be distributed with each male receiving twice a female's share. And if they are all related through the mother, they will distribute the estate with a male receiving twice the share of a female.
And if they exist together and are related to the deceased from either side --such as if there are with the maternal grandparents full or agnate brothers and sisters-- the grandfather or the grandmother or both together will inherit one-third and the brothers and sisters two-thirds.
And if the paternal grandparents exist along with uterine brothers and sisters, a sole uterine brother or sister will receive one-sixth; if they are more than one, they will be entitled to one-third, distributed equally among males and females, with the remainder going to the grandparents who distribute it with the grandfather receiving twice the share of the grandmother.
'Children' how low soever, of brothers and sisters of all kinds, represent their parents in their absence while inheriting along with all kinds of grandparents, each one of them inheriting the share of the person through whom he or she is related.