What is milk kinship in Islam?

What the Quran and Sunnah say

When the Quran lists the women a man can't marry, it includes nursing relatives alongside blood relatives:

The Prophet ﷺ explained that this goes further than mother and sister. About a girl whose father had shared a nurse with him, he said, "foster relations are treated like blood relations (in marital affairs). She is the daughter of my foster brother" (Sahih al-Bukhari 2645). So milk aunts, uncles and nieces are covered too.

How many feeds, and at what age?

Scholars agree it only counts when the baby is young, within the first two years for most scholars. They differ on how much feeding is needed:

• Shafi'i and Hanbali: five separate, full feeds. Aisha reported that the rule settled on "five sucklings" (Sahih Muslim 1452), and the Prophet ﷺ said "one suckling or two do not make (marriage) unlawful" (Sahih Muslim 1450).
• Hanafi and Maliki: any amount of milk that reaches the baby within the nursing age creates kinship.

Because of this difference, families usually keep track of who nursed whom, and how often, so there's no confusion later.

Who becomes mahram

The nursed child becomes like a son or daughter to the nursing woman and, according to most scholars, to her husband (the "milk father"). Her children, older or younger, become the child's milk siblings. The child's own biological siblings, however, aren't affected. Milk kinship affects marriage and mahram status only. It doesn't give inheritance rights or change the child's name or lineage.

In practice this means a woman doesn't need hijab in front of her milk son or milk brother, and they can't marry. For how mahram rules work in mixed families, see Do I need to wear hijab in front of my non-Muslim family?

Why it matters today

Milk kinship comes up most with adoption. Islam doesn't allow changing an adopted child's lineage, so an adopted boy isn't automatically mahram to his adoptive mother once he grows up. If the adoptive mother, her sister or her daughter nurses the baby before age two, the child becomes a mahram member of the family, which makes hijab and home life much easier. See Can I adopt a child as a Muslim?

Donor milk banks raise questions too, since the donors are often unknown. Scholars differ, and many advise asking a local scholar before using pooled donor milk.