Shaykh al-Islām—may Allah have mercy on him—was asked:

“A man proposed to a female relative, but her father said, ‘She was breastfed with you,’ and forbade him to marry her. After his own father died, he married her. Trustworthy witnesses had attested that her mother acknowledged having breastfed him. Later, however, she denied it, saying, ‘I only said that to serve a purpose.’ Is marriage to her lawful?”

He replied:

If the mother is known to be truthful and states that she breastfed him on five occasions, her statement is accepted. If he has married her daughter, the couple must be separated, according to the more correct of the two scholarly opinions. As established in Sahīh al-Bukhārī:

“The Prophet, peace and blessings be upon him, ordered ‘Uqbah ibn al-Hārith to separate from his wife when a black slave woman stated that she had breastfed them both.”

If, however, there is doubt about her truthfulness or the number of nursing occasions, the marriage is a doubtful matter, and avoiding it is preferable. Nevertheless, no ruling separating the couple is issued without evidence requiring it.

If she retracts her testimony before the marriage, marriage to the woman is not forbidden. But if it is known that her retraction is false and that she retracted because he had prevailed upon her to conceal her testimony, then the marriage is not lawful. And Allah knows best.