Is it permissible to marry the daughter of one's paternal uncle's son, knowing that the paternal uncle's wife breastfed the questioner and the girl's brother?
If a man is suckled five known times by a woman, this woman becomes his foster mother through suckling (rada’a), and he becomes a brother to her daughters, an paternal uncle to her sons' daughters, and a maternal uncle to her daughters' daughters. Consequently, it is forbidden for him to marry the daughters of that woman, her sons' daughters, and her daughters' daughters, based on the words of Allah the Almighty: {Forbidden to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother’s daughters, and your sister’s daughters}, and based on the words of the Prophet, peace be upon him: "What is forbidden by lineage is also forbidden by suckling." Therefore, marrying the daughter of your foster sister is forbidden.
Summarized from the full answer at Ftawy · reviewed Sep 2, 2026
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