Will the girl engaged to our brother become unlawful for us (as a paternal aunt) because my mother and her mother shared suckling from her mother's father?

If the suitor's mother was suckled by the fiancée's stepmother five separate times within the first two years of life, then the suitor's mother becomes a daughter to the wet-nurse and her husband. Consequently, the wet-nurse's children become siblings to the suitor's mother, and thus maternal uncles and aunts to the suitor's children. Since "what is prohibited by lineage is prohibited by fosterage," marriage to a sister's daughter is forbidden.

However, if the suckling was from a woman who is not a stranger to the fiancée's father's wife and not her mother, there is no prohibition as long as the suitor or his mother did not suckle from the same breast as the fiancée or her mother. The mere fact that the suitor's mother suckled from the same breast as the fiancée's siblings from a woman who is not the fiancée's father's wife does not prohibit the suitor from marrying this girl.