He replied:

Praise be to Allah. If the wet nurse breastfed her five times within her first two years, she became her daughter. All the wet nurse’s children are therefore forbidden to this breastfed girl in marriage, whether born before or after the breastfeeding. This is agreed upon by the Muslims. Whoever regards this as lawful must be called upon to repent; if he does not repent, he is put to death. However, if the breastfed girl has biological sisters, they may marry her sisters through breastfeeding, by agreement of the Muslims. Allah knows best.

He—may Allah have mercy on him—was asked:

“A man married a second wife after his first wife. A boy had been breastfed by the first wife, and the father has a daughter by the second. May the breastfed boy marry this daughter? If he marries her and consummates the marriage, must they be separated? Is there any disagreement among the imams concerning this?”

He replied:

If he received the breastfeeding that establishes a prohibition of marriage, he may not marry this daughter according to the schools of all four imams, without any disagreement among them, because the milk is attributed to the husband. Ibn ‘Abbās was asked about a man with two wives, one of whom breastfed a boy while the other breastfed a girl: “May the two marry one another?”

He replied: “No. The man responsible for the pregnancies is the same.”

The basis for this is ‘Ā’ishah’s hadith, recorded in both Sahih collections. She said:

“Aflah, the brother of Abū al-Qu‘ays, asked permission to enter my presence. The wife of Abū al-Qu‘ays had breastfed me. I said, ‘I will not give you permission until I ask the Messenger of Allah.’ I then asked him—may Allah bless him and grant him peace—and he said, ‘He is your uncle, so let him enter your presence. It is forbidden