Thus, the Quran indicates that if the deceased’s only heirs are his mother and father, or his mother and a residuary heir other than the father or a son, his mother receives one-third. This is an inference from the weaker case to the stronger. The son, however, has a stronger claim than the father, so she receives one-sixth alongside him.

When an heir entitled to a prescribed share inherits alongside the residuary heirs, the mother has been allotted one-sixth in the presence of daughters or sisters. As for a single sister inheriting with the mother, the mother receives one-third alongside a male sibling, so she is all the more entitled to it alongside a female sibling. Her share is reduced from one-third to one-sixth only by the presence of siblings; one sibling does not constitute a plurality of siblings. Since she receives one-third alongside a single brother, she is all the more entitled to it alongside a paternal uncle or another such heir.

There is disagreement concerning the grandfather. A view is reported from Ibn Mas‘ūd, while the majority hold that she receives one-third of the estate alongside the grandfather. This is correct, because the grandfather is more distantly related to the deceased than she is, and the father excludes him from inheritance. He therefore cannot deprive her of any part of her entitlement. Strict analogy places the father in relation to the mother as the daughter in relation to the son, and the sister in relation to the brother: each pair consists of a male and a female of the same class, both being residuary heirs. Likewise, the wife has been allotted half the share allotted to the husband, because they are a male and a female of the same class.

As for what the Book indicates concerning the mother’s inheritance, Allah says:

“Each parent receives one-sixth of what he leaves if he has children. If he has no children and his parents inherit from him, his mother receives one-third.”

Allah Most High prescribed this share for her on two conditions: that the deceased have no children, and that his father inherit from him. This indicates that she is not invariably given one-third whenever there are no children.