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

“Two sisters each have a child: one has a son and the other a daughter. The boy’s mother breastfed the girl, but the girl’s mother did not breastfeed the boy. Later, one sister had daughters and the other sons. May the brother of the breastfed child marry the girl who was nursed on the same milk as his brother? Likewise, may the children of one sister marry the children of the other, apart from the two children involved in the breastfeeding?”

He replied:

Praise be to Allah. The girl who was breastfed may not marry any of her nursing mother’s children, whether they were born before or after the breastfeeding. As for the breastfed girl’s siblings, they may marry whichever of the nursing mother’s children they wish. Thus, someone who was not breastfed by a woman may marry one of her children, but he may not marry any of the children of a woman who breastfed him.

If a boy is breastfed by the mother in question, or a girl from among the children in question is breastfed, neither may marry any of the other woman’s children. However, the siblings of either of the two children who shared the nursing may marry the siblings of the other, provided neither prospective spouse was breastfed by the other’s mother. The marriage prohibition applies only to the child who was breastfed, not to that child’s siblings who were not breastfed by that woman. Nevertheless, all the nursing mother’s children are forbidden in marriage to the child she breastfed. And Allah knows best.