Breastfeeding Between the Children of Two Brothers and the Ruling on Marriage

Question:
My daughter was breastfed by my brother’s wife, and my brother’s daughter was breastfed by his wife. My daughter was breastfed by my brother’s wife, and my brother’s daughter was breastfed by my wife. Is it permissible for my brother’s other sons to marry my other daughters, and vice versa?

Answer:
If a man’s brother’s child is breastfed by your wife, through a lawful breastfeeding of five times or more, then this breastfeeding makes her forbidden to his sons, and she becomes their sister. If she was breastfed within the first two years, five times or more, by your wife, then none of your sons may marry her. However, there is no harm in his marrying her sisters who were not breastfed by his wife, the other ones. Likewise, if his daughter was breastfed by his brother’s wife, she becomes his brother’s daughter, provided the breastfeeding was five times or more within the first two years. It becomes forbidden for his sons to marry her, but there is no harm for his brother’s sons to marry his other daughters, other than the one who was breastfed by his brother’s wife. This should be understood: the one who was breastfed by his brother’s wife is not permissible for your sons, and the one who was breastfed by his brother’s wife is not permissible for his sons. The one of your daughters who was breastfed by his brother’s wife, and the one of your brother’s daughters who was breastfed by his wife, is not permissible for your sons either. The point is: your brother’s daughter who was breastfed by your sons becomes his daughter, and is forbidden to your sons. And your daughter who was breastfed by his brother’s wife, and became his daughter, is not permissible for his sons. As for the others, there is no harm. May Allah guide everyone, and may Allah send prayers and peace upon