The Milk is Attributed to the Man

Question :

Allah has forbidden us to marry a sister through breast- feeding, but is the daughter forbidden to the son of the breast-feeding woman who was born before the daughter and is older than she? And if a man has two wives and another boy was fed by one of them, are the daughters of both wives forbidden to him? And how many times must the child be breast-fed in order for prohibition to occur?

Answer:

If a person is breast-fed by a woman in a manner which brings about prohibition, he is considered a son to her through breast-feeding and a brother to all of her children, male and female whether they were already born when the breast- feeding took place, or they were born after he was suckled. This is based upon the generality of the Words of Allah, Most High:
"your foster milk suckling sisters."

If a person was breast-fed in a manner which results in prohibition by one of a man's two wives, then all of the children of that man are brothers and sisters through breast-feeding to that person who was suckled. It does not matter whether the breast-feeding was from one of his wives, or from both of them. This is because the milk is attributed to the father.

The breast-feeding which causes prohibition is that which occurs five times or more during the first two years of life. One breast-feeding is when the child sucks the breast and then leaves it whether he drank all of the milk, or only sucked a little from it.

Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 6 Page 153