Someone who has not examined the matter carefully may therefore say: “Blood kinship forbids my brother to marry my mother, whereas breastfeeding does not impose the corresponding prohibition.” This is mistaken. The breastfeeding counterpart of what is forbidden through blood kinship is for his sister or brother through breastfeeding to marry this brother’s son or his mother through breastfeeding. For example, if he and another child were breastfed by one woman, with the milk attributable to one man, his sister through breastfeeding is forbidden to marry his brother or sister through breastfeeding, since both are siblings of the breastfed child. Likewise, neither of them may marry his father or mother through breastfeeding, because they are those parents’ children through breastfeeding—not because they are siblings of their children. Whoever reflects on this and similar cases will have the confusion dispelled.

As for “breastfeeding an adult,” it does not establish marriage prohibitions according to the four imams. Such prohibitions arise only from breastfeeding a young child, such as one breastfed within the first two years. The imams disagree concerning a child breastfed close to the two-year limit, but the position of al-Shāfiʿī and Ahmad is that this does not establish marriage prohibitions.

An adult man and an adult woman do not become forbidden to one another in marriage through the breastfeeding of relatives. For example, if his wife breastfeeds his biological brother, his wife does not thereby become forbidden to him. As explained above, he may marry a woman who is his biological brother’s sister through breastfeeding, since neither blood kinship nor breastfeeding connects him to her. His wife becomes forbidden only to his brother, because she is his brother’s mother through breastfeeding, not his own. The breastfed child’s mother through breastfeeding does not become the mother of his biological siblings, because she breastfed that child alone, not the others.

However: