The Issue of labn al-faḥl (paternal milk affiliation)

Question: A child grew up in his uncle’s home and was breast-fed by the uncle’s wife. Some time later the uncle married a second woman, who then bore him daughters. May that child marry one of the second wife’s daughters, even though he nursed only from the first wife? Kindly advise us, and may Allah reward you.

Answer: He may not. It is impermissible for him to marry any of the second wife’s daughters, for she is a co-wife of the woman who nursed him and both women share the same husband. This is the issue known as labn al-faḥl—the milk that is ascribed to the husband—and the sound ruling is that such a marriage is forbidden. The daughters of the other wife are his milk-sisters through their common (milk) father. Allah knows best.
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The breastfeeding prohibition applies only to the nursing child and his descendants

Question: My sister nursed her firstborn son together with my own firstborn. Do the girls she bore after that son become unlawful in marriage to my sons who were born after my eldest? Please advise, and may Allah reward you abundantly.

Answer: Yes. Your sister’s son whom you nursed becomes your milk-son, and therefore your daughters are forbidden to him, for he has become their milk-brother. His own brothers, however, may marry your daughters, and likewise your sons may marry the sisters of the nursling—your sister’s other children—because they have no connection to that breastfeeding. The prohibition created by breastfeeding is confined to the nursling himself and to his line of descent.