What is the share of a son's daughter (or daughters) when co-inheriting with a primary daughter (or daughters)?
General Chapter
Al-Mughni
Book of Inheritance Shares (Farā'id)
Primary text
When one daughter or multiple daughters are present with a son's daughter or multiple son's daughters, the primary daughter receives one-half (1/2), and the son's daughter(s) receive one-sixth (1/6) to complete the two-thirds share designated for daughters. This is universally agreed upon by scholars. The basis for this is the verse: {But if they are [only] daughters, two or more, for them is two-thirds of what he leaves} (Quran 4:11). Since both primary daughters and son's daughters are categorized as female offspring, they are entitled to the two-thirds share by the decree of the Book, and they do not exceed it. The primary daughter is specifically allotted half (1/2) due to her direct relation and fixed prescription, leaving the remainder for the others to complete the two-thirds. Therefore, jurists state they receive one-sixth (1/6) to complete the two-thirds.
Supporting text
This is supported by the narration where Abu Musa was asked about a daughter, a son's daughter, and a sister. He gave the daughter half and the rest to the sister. When informed of this, Ibn Mas'ud repudiated Abu Musa's ruling, stating he would rule according to the ruling of the Messenger of Allah (PBUH): half for the daughter, one-sixth for the son's daughter to complete the two-thirds, and the remainder for the sister. This report is established in consensus.