[A Woman Breastfed Her Husband’s Minor Wife]

Answer: The minor wife, by that breastfeeding, becomes the man’s daughter, and the major wife becomes the mother of his minor wife. Thus, the marriage to the major is dissolved, because she became the mother of his wife, and the marriage to the minor is dissolved, because she became his daughter.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1