Shaykh al-Islām—may Allah have mercy on him—also said:
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If a child receives five breastfeeds from a woman before reaching two years of age, he becomes her child. He may therefore not marry any of her children, whether born before or after she breastfed him. Her husband, through whom she became pregnant and produced milk, becomes his father, and he may not marry any of that man’s children. Thus, if his wife breastfeeds a boy and a girl five times each, they may not marry one another; rather, they are siblings. And Allah knows best.
He—may Allah have mercy on him—was asked:
“A woman breastfed a boy on two occasions. Ten years later, she became pregnant and gave birth to a girl, and the boy had become a young man. May he marry that girl or not?”
He replied:
If she breastfed him five times within his first two years, he has become her son. He may not marry any of that woman’s children, whether she gave birth to them before or after breastfeeding him, by scholarly consensus.