Forty years ago, a person was breastfed by my mother five sure times. Now he wants to marry my paternal half sister, who is not a daughter of the foster-mother
but a daughter of another wife (of my father). Please answer me, may Allah reward you with the best.
The breastfeeding that prohibits marriage takes place only if the baby suckles five times or more during the first two years of breastfeeding. Every time a baby holds the breast and sucks milk and then leaves it to take a breath or to suck the other breast, this is counted as one time of suckling until five times of suckling are counted. If the number is less than five times, even if they are four times of suckling, this breastfeeding does not prohibit marriage. Mentioning the women prohibited to marry, Allah (Exalted be He) says,
...your foster mothers who gave you suck, your foster milk suckling sisters.
It is reported by Al-Bukhari and Muslim that the Prophet (peace be upon him) said,
Milk relationships are equal to blood relationships in rendering marriage unlawful i.e., whatever is prohibited due to blood relationships is also prohibited by milk relationships.
It is related by
`Aishah (may Allah be pleased with her) said:
Amongst what was sent down of the Qur'an was 'ten known times of suckling forbid marriage.' Then it was abrogated by 'five known times of suckling.' The Messenger of Allah (peace be upon him) died while the latter rule was in force.
It is also related on the authority of
`Aishah (may Allah be pleased with her) that Allah's Messenger (peace be upon him) said:
One suckling (Part No. 21; Page No. 89) or two do not make (marriage) unlawful.
Thus if he was breastfed by your mother five times, as you have mentioned in your question, and if your mother was the wife of your father during the breastfeeding, then this baby becomes a milk son to both your mother and father, and a brother by breastfeeding to all sons and daughters of your father whether they are from your mother or his other wife. Accordingly, it is not permissible for him to marry your paternal half sister, since she is his paternal half sister by breastfeeding. An exception to this case is that if he was breastfed by your mother while she had been married to another husband before being married to your father. In that case there would be nothing to prevent him from marrying your paternal half sister, since in this case, he is not a son to your father or a milk brother to your paternal half sister.May Allah grant us success. May Allah's Peace and Blessings be upon our Prophet and upon his family and Companions!