How can a woman marry her son's brother?

A woman is not permitted to marry her son's brother by lineage, as he is either her son or her husband's son. However, by suckling, this is possible. So, if Fatimah suckled Zayd but did not suckle his brother Amr, and there was nothing between her and Amr that would prohibit her to him, then it is permissible for Amr to marry Fatimah, even though she is his brother's mother through suckling. Exceptions to the generality of the hadith "What is prohibited by lineage is prohibited by suckling" include women who are prohibited by lineage but not by suckling, such as the woman who suckled one's brother or sister.