There is a difference of opinion on this, which has implications if a virgin were to nurse a child: would he become her foster child, such that she would be forbidden to him in marriage, or not?

The well-known position in the madhhab is that the milk must have accumulated as a result of pregnancy, and that a virgin’s nursing has no legal effect because it did not result from pregnancy. The correct view, however, is that it does have legal effect even if it did not result from pregnancy, due to the general wording of the verse: “And your mothers who nursed you,” and because the meaning is clear: this child has been nourished by the milk.

The evidence that nursing has an effect in rendering marriage unlawful is Allah’s saying: “And your mothers who nursed you and your sisters through nursing” [al-Nisa’ 4:23]. Thus, the verse mentions two categories, or types, while the sunnah completes the remainder. The Prophet, peace and blessings be upon him, said: “Nursing makes unlawful whatever blood relationship makes unlawful.” (1)

His statement, “Except the mother of one’s sister and the sister of one’s son”: the mother of one’s sister through nursing is not forbidden to him, whereas the mother of one’s sister by blood is forbidden to him. This is because the mother of your sister by blood is either your own mother or your father’s wife. If she is your father’s wife, she is forbidden to you through marriage. But the mother of your sister through nursing is not your father’s wife; rather, she is the wife of another man. Thus, if your sister nursed from a woman, it is permissible for you to marry that woman.

Likewise, the sister of your son through nursing is like the mother of your sister through nursing. She would be forbidden to you if she were related to you by blood, but through marriage ties rather than blood kinship. This makes clear that this exception has no basis and is unnecessary,