Marriage to the grand-daughters of one’s wet-nurse
Question: I was breast-fed by a woman who now has grand-daughters. May I marry any of them—that is, the daughters of her daughters? Please advise me, and may Allah reward you.
Answer: If you nursed from that woman a complete, legally-recognized breastfeeding—five distinct sucklings within the first two years of life—then you are counted as her son through breastfeeding. Her daughters are therefore your milk-sisters, and their daughters are the daughters of your milk-sisters. In relation to them you are their maternal milk-uncle, so it is not permissible for you to marry any of those grand-daughters, i.e., the daughters of her daughters, because they are your milk-sisters’ daughters and you are their uncle. Allah knows best.
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Breastfeeding one’s full brother
Question: A woman breast-fed her full brother. What is the Islamic ruling on marriage between their respective children?
Answer: When a woman breast-feeds her own full brother, he becomes her son through breastfeeding, and his children become her children; she is their grandmother through milk-kinship. Consequently, her biological children are the milk-siblings of the nursed brother and, in turn, uncles to his children. Therefore, marriage between the two sets of children is impermissible in this situation: the wet-nurse’s children are uncles to the children of the nursed infant, and the latter are her grandchildren through breastfeeding—that is, she is their grandmother through milk-kinship.