Source: Fatawa Aman Puri by Sheikh Ghulam Mustafa Zaheer Aman Puri
Question:
The mother-in-law said to the son-in-law that I breastfed you in childhood, so what is the ruling?
Answer:
If the mother-in-law confidently testifies that she has breastfed her current son-in-law at least five times during the period of breastfeeding, then the mother-in-law's testimony will be valid, breastfeeding will be established, and separation will be arranged between the husband and wife, because they are both foster siblings. Just as marriage between biological siblings is forbidden, so is marriage between foster siblings. In this regard, the testimony of a single woman is sufficient.
❀ Sayyiduna Uqbah bin Harith (may Allah be pleased with him) narrates:
"I married the daughter of Abu Wahab (Umm al-Banī), and a black woman came and said: I have breastfed both of you. I asked the Prophet (peace be upon him) about this, and he turned away from me. I asked again, and he turned away again. The third or fourth time he said: When this has been said, how can she remain with you? So he prohibited her from (staying with) her (wife)."
Reference: (Sahih al-Bukhari: 2659)
