Allah (Exalted be He) prohibits one to marry his milk sister; is a person forbidden to marry the girl that nursed from his mother when he is born earlier and is older than her?
If a man has two wives and a boy is breastfed by one of them, would the daughters of the two wives be forbidden to him? What is the standard number of suckles that establishes prohibition?
If someone is breastfed by a woman the breastfeeding that renders marriage unlawful, he shall be considered her milk son and a brother of all her male and female children, regardless of whether they are born at or after the time of breastfeeding. This is based on the generality of His (Exalted be He) Saying,
...your foster milk suckling sisters
If someone is breastfed by one of a man's wives in a manner that renders marriage unlawful, all the children of this man should be considered milk-brothers and milk-sisters to the breastfed person, whether he is breastfed by one of or the two wives. That is because the man is the main cause of milk. The breastfeeding that renders marriage unlawful is that of five times or more during the first two years, bearing in mind that one time of breastfeeding is counted when the baby holds the breast, suckles milk, and leaves it to take a breath or to move. (Part No. 21; Page No. 7) May Allah grant us success. May peace and blessings be upon our Prophet, his Family, and Companions.