The World Federation One Stop Fiqh

Ruling 2490

If a woman fully breastfeeds her daughter’s child, the daughter becomes unlawful (ḥarām) for her own husband. The same applies if she breastfeeds the child of her daughter’s husband from another woman. However, if a woman breastfeeds her son’s child, her son’s wife – who is the mother of that breastfeeding child – does not become unlawful for her husband.