He Married His Sister Through Breast-feeding in Ignorance

Question :

It became apparent to me after having sex with my wife that she was my sister through breast-feeding, because I was breast-fed with her sister, so is she forbidden to me in this situation?

Answer:

Yes, if the matter is as you have stated, that you were breast-fed with the wife's sister by her mother- meaning that you were breast-fed by the wife's mother, or by her father's wife invalid. then you are, in this case, a brother and the contract is invalid.

But you must know that there is no effect from breast-feeding unless it took place five times or more during the first two years of life, before weaning. If it was less than this, then there is no effect from it, and prohibition is not brought about. Then if you are certain that you were breast-fed five times or more during the first two years by the mother of the woman you married, the two of you must part, due to the marriage being invalid. As for any children born before you knew, they are legally attributed to you, because those children were created from a fluid (i.e. sperm) through sexual intercourse which took place in ignor- ance, and when sexual intercourse takes place due to ignorance, kinship of the child is attributed to the father, as the scholars have said.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Page 127