What is Unlawful by Reason of Lineage is Unlawful be is Unlawful by Reason of Fostering

Question:

I inform your Eminence that my mother was married to a man before my father, she bore him a son. She breast-fed a sister of hers along with this boy, and the breast-feeding lasted for around a week. Then my mother was separated from this man and she was taken (in marriage) by my father. Is it permissible for us, the sons of the second man, to marry from the daughters of our maternal aunt who was breast-fed by our mother, or not?

Answer:

It is not permissible for you to marry the daughters of this maternal aunt of yours, because by the aforementioned fostering, she has become a sister to you and you have become maternal uncles to her children; and it has been authentically reported from the Messenger of Allah Sallallahu alaihhi wasallam that he said: What is unlawful by reason of lineage is unlawful by reason of fostering.
Source:
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Darussalam Vol:5 no.234

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