The honorable Shaykh, I wish to marry my cousin, knowing that my older brother has nursed from my aunt more than once, whereas I have not nursed from her.

The questioner : The honorable Shaykh, I wish to marry my cousin, knowing that my older brother has nursed from my aunt more than once, whereas I have not nursed from my aunt at all, and my cousin has not nursed from my mother at all. So my question is: Is it permissible for me to marry my aunt's daughter, or have I become her brother? Please inform us of this, may Allah reward you with good.

The Shaykh : The answer to this question is derived from the saying of the Prophet (peace and blessings be upon him): This means that breastfeeding makes forbidden what is made forbidden by blood kinship, because lineage is kinship. I have a comment on this term coming up shortly, if Allah wills. Based on this hadith, you are permitted to marry your aunt's daughter, whose sister nursed from her mother, because there is no connection between you and her. You are not her brother because you did not nurse from her mother, and she did not nurse from your mother. Prohibition only occurs between the one who nurses and his descendants. I mean that breastfeeding only affects the one who nurses and those who branch off from him among his descendants. As for those who are in his position regarding siblings, or who are higher than him among ancestors, the prohibition does not extend to them.
The prohibition extends from the one who nurses to him and his descendants, considering the woman who nursed him and those to whom her milk is attributed. That is, the one who nursed him becomes his mother, her mother becomes his grandmother, her father becomes his grandfather, her brothers become his maternal uncles, and her sisters become his maternal aunts. Likewise, the one to whom the woman's milk is attributed, whether her husband, her master, or one who had intercourse with her under doubt, becomes the father of the nursed child. His children become brothers to the nursed child, his brothers become paternal uncles, and his sisters become paternal aunts. We take all this from the saying of the Prophet (peace and blessings be upon him): .
What I promised you earlier regarding the term "lineage" is that many common people do not understand by the term "lineage" or "kinship" anything other than the relatives of the husband and wife. So a man might say, "These are my relatives," because he married into them. This is incorrect linguistically. Lineage refers to kinship through the father or through the mother, and kinship is likewise kinship through the father and through the mother. As for the relatives of the spouses, they are called in-laws, not lineage. Allah the Exalted said: Allah the Exalted established the connection between humans through these two matters: lineage and in-law relations. They are two divisions, meaning that one is a division of the other and distinct from it. I wanted to point this out so that people know the meanings of the legal terms and do not err in them.