The breastfeeding that prohibits marriage takes place only if a baby had five sucklings or more during the first two years. The description of one suckling is as follows: a baby holds a breast and sucks milk. If it leaves the breast to take a breath or to be moved to another breast, then this is to be counted as one suckling, and so on until five sucklings are counted. If the number of sucklings is less than five, even four, breastfeeding does not prohibit marriage. Mentioning Mahrams (spouses or unmarriageable relatives), Allah (Exalted be He) says:
Forbidden to you (for marriage) are: your mothers
...your foster mothers who gave you suck, your foster milk suckling sisters.
The mothers shall give suck to their children for two whole years, (that is) for those (parents) who desire to complete the term of suckling
(Part No. 21; Page No. 114) 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.
(Agreed upon by Imams Al-Bukhari and Muslim)
It was also authentically reported that
`Aishah (may Allah be pleased with her) said: "It was among the revelation of the Qur'an that ten known times of breastfeeding make the marriage prohibited, but this was later abrogated and reduced to five. The Messenger of Allah (peace be upon him) died and the ruling remained effective." On the other hand, if he was breastfed by her less than five times, or if he was breastfed at any time other than the first two years (of his life), it is permissible for him to marry her. As for you, it is permissible for you to marry her, regardless of whether the breastfeeding of your brother exceeded this number or not.May Allah grant us success. May peace and blessings be upon our Prophet, his family and Companions!