Some of the scholars of the Sunnah said:
“The brothers of the Prophet’s wives are not to be called ‘the maternal uncles of the believers’; for if that title were used for them, their sisters would likewise be called ‘the maternal aunts of the believers.’ If they truly were their maternal uncles and aunts, it would be forbidden for any believer to marry his maternal aunt (1), and forbidden for any believing woman to marry her maternal uncle.
Yet it is established by the revealed text and scholarly consensus that male and female believers may marry the sisters and brothers of the Prophet’s wives, respectively (2). Thus, al-Abbās married Umm al-Fadl, the sister of Maymūna bint al-Hārith, the Mother of the Believers (3), and Abd Allah, al-Fadl, and others were born [to him] by her (4). Likewise, Abd Allah ibn Umar, Ubayd Allah, Muāwiya, Abd al-Rahmān ibn Abī Bakr, and Muhammad ibn Abī Bakr married the believing women whom they married (5). Had these men been their maternal uncles, no woman could lawfully marry her maternal uncle.”
They said:
“Likewise, the mothers of the Prophet’s wives are not called ‘the grandmothers of the believers,’ nor are their fathers (6) called ‘the grandfathers of the believers.’ This is because not all the rulings of blood kinship apply to the Mothers of the Believers; only the sanctity owed to them and the prohibition against marrying them are established. The rulings of blood kinship are divisible: suckling (7), for example, establishes the prohibition of marriage and unmarriageable kinship, but does not establish all the other rulings of blood kinship. All of this is agreed upon.”
Those who applied the title “maternal uncle of the believers” to any one of those men did not dispute (8)