The origin of milk kinship is the couple, not the woman alone
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Question: A child was raised in his uncle's house and nursed from his uncle's first wife. After a period, his uncle married a second wife, who bore him a daughter. Is it permissible for this child (when he grows up) to marry his cousin from the wife from whom he did not nurse?
Answer: If the child mentioned nursed from his uncle's wife five times or more while he was within the two-year period, then he becomes his uncle's son through milk kinship, and all of his uncle's children from all his wives become his siblings through milk kinship—both males and females.
And thus you know: it is forbidden for the child mentioned to marry the daughter mentioned, because she is his sister through his father via milk kinship—if the situation is as we have described. And Allah the Exalted said in His clear Book, when He mentioned the prohibited women: [al-Nisa: 23], and the Prophet (peace and blessings be upon him) said: Sahih Muslim 1445i It is agreed upon that it is authentic.
Answer: If the child mentioned nursed from his uncle's wife five times or more while he was within the two-year period, then he becomes his uncle's son through milk kinship, and all of his uncle's children from all his wives become his siblings through milk kinship—both males and females.
And thus you know: it is forbidden for the child mentioned to marry the daughter mentioned, because she is his sister through his father via milk kinship—if the situation is as we have described. And Allah the Exalted said in His clear Book, when He mentioned the prohibited women:
your [milk] mothers who nursed you, your sisters through nursing
وَأُمَّهَاتُكُمُ اللاَّتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِon account of fosterage as one is Mahram on account of consanguinity
يحرم من الرضاعة ما يحرم من النسب
Source
binbaz.org.sa
The Arabic text is copied verbatim from the original source, without any edits.