Shaykh al-Islam said: “It is unanimously agreed that whatever is forbidden through lineage is likewise forbidden through nursing. Thus, if an infant nurses from a woman five full times within the first two years, before weaning, he becomes her child by consensus, and—by the agreement of the famed Imams—the child of the owner of the milk. All of their children, therefore, become siblings to the nursling, whether they are related to him through the father alone, the mother alone, or through both.

There is no difference—by unanimous agreement—between the woman’s children who nursed alongside the infant and those born to her before or after the nursing.

Accordingly, all of the woman’s relatives become relatives of the nursling through nursing: her children are his brothers and sisters; the children of her children are like the children of his siblings; her fathers and mothers are his grandfathers and grandmothers; and her brothers and sisters are his maternal uncles and aunts. All of these are forbidden for him to marry.

The daughters of those uncles and aunts through nursing, however, are permissible for him, just as their counterparts through lineage are permissible. The same ruling applies to the man’s relatives and his milk relatives, and the children of the nursling occupy the same position.

As for the nursling’s own siblings—whether by lineage or by nursing from a woman other than this wet-nurse—they are strangers to her relatives, so those siblings may lawfully marry the wet-nurse’s children. All of this is unanimously agreed upon by the scholars.”

Shaykh Taqī al-Dīn held that the prohibitions of marital affinity (muṣāhara) are not established through nursing. Therefore, it is not unlawful for a man to marry the mother of his wife through nursing, or the daughter of his wife through nursing if the milk belonged to someone else; nor is it unlawful for a woman to marry the father of her husband through nursing. A consensus, however, has been reported contradicting the shaykh’s view.

Resolution of the International Islamic Fiqh Academy concerning milk banks: Resolution No. (6):

“In the name of Allah, the Most Compassionate, the Most Merciful. All praise is due to Allah, Lord of the worlds, and may blessings and peace be upon our Master Muhammad, the Seal …”