That Which is Forbidden Due to Kinship is Forbidden Due to Breast-feeding 2

Question :

My wife and my brother's wife gave birth to children and each of them breast-fed the children of the other; so is it permissible for my brother's sons to marry my daughters and vice versa?

Answer:

The breast-feeding which causes prohibition is that which occurs five times or more during the first two years of life - this is what is proven by the legal evidences from the Book (of Allah) and the Sunnah. One breast-feeding is when the child. takes the breast and sucks milk from it, then he leaves it to take a breath or to transfer (to the other breast) or some such reason. Then if he returns (to the breast), it is another breast-feeding, and so on. Based upon this, if any son of yours was breast-fed by your brother's wife in the manner described above, it is not permissible for him to marry your brother's daughters, because he will be a brother to them through breast-feeding. Allah, Most High has said:
"Forbidden to you (for marriage) are: your mothers."

Up to His Words:
"your foster milk suckling sisters."

The same is said of your brother's sons, regarding your daughters.

However, if the breast feeding occurred less than five times, or it occurred after the first two years of life, then this does not prevent marriage. And if any daughter of yours was breast-fed by your brother's wife in the manner described above, then it is not permissible for any of your brother's sons to marry her, because she is his sister through breast-feeding. Likewise, it is not permissible for those of your brother's daughters who were breast-fed by your wife to marry any of your sons.

Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 6 Pages 148-149