[The Ruling on a Grandmother Offering Her Breast to Soothe an Infant 1]

The answer I gave was: the nursling becomes the grandmother’s child, and so it is not permissible for him thereafter to marry his cousin-girl and the like. This is because a woman, even in advanced age, may, upon suckling, excrete milk or serum. If the grandmother—or anyone else—knows that something of that entered the child’s stomach, the matter is clear; and if she does not know, then believers are cautious in doubtful matters—especially where private parts (marriageability) are concerned. And in the hadith: “How is it with him when it has been said?”
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1