The evidence that a full sister receives one-half when she is alone, and that two or more full sisters receive two-thirds, is the saying of Allah, Exalted is He:
“They ask you for a ruling. Say, ‘Allah gives you a ruling concerning one who leaves neither ascendants nor descendants: if a man dies leaving no child but has a sister, she shall have one-half of what he leaves.’”
That is, there is no inheriting descendant; nor is there an inheriting male ascendant.
“But if there are two sisters, they shall have two-thirds of what he leaves.” [al-Nisa: 176]
The evidence that a son’s daughter, alongside a daughter, receives one-sixth is the saying of Allah, Exalted is He:
“Allah instructs you concerning your children: the share of the male is equal to that of two females. But if there are only daughters, two or more, they shall have two-thirds of what he leaves.” [al-Nisa: 11]
Thus, when the daughter takes one-half, one-sixth remains to complete the two-thirds. Allah has not prescribed more than two-thirds for female descendants. Therefore, when dividing the estate, we must say: the daughter receives one-half, and the son’s daughter receives one-sixth, completing the two-thirds. Likewise, with full sisters and paternal half-sisters, the full sister receives one-half and the paternal half-sister receives one-sixth. The evidence is the saying of Allah, Exalted is He:
“But if there are two sisters, they shall have two-thirds of what he leaves.” [al-Nisa: 176]
He mentioned no share beyond this. This, praise be to Allah, is clear evidence.
A case arose during the time of Abdullah ibn Masud, may Allah be pleased with him: a daughter, a son’s daughter, and a full sister. They came to seek a legal ruling from Abu Musa al-Ashari, Abdullah ibn Qays, may Allah be pleased with him, and said: “A man has died leaving a daughter, a son’s daughter, and a full sister.” He replied: “The daughter receives one-half, and the full sister receives the remainder, because full sisters become residuary heirs alongside daughters.” He then said to the questioner: “Go to Ibn Masud; he will agree with me on this,” in order to verify the matter.