Is it permissible for the questioner’s daughter to marry her nephew, the son of her brother, whom her mother did not breastfeed, given that the questioner’s mother breastfed two of his sisters from his mother, and her brother’s wife breastfed two of the questioner’s brothers?

If a boy or a girl is suckled by a woman, he or she becomes her child and her husband's child through suckling. This applies specifically to the suckling child, not to his or her siblings. The Malikis state: "The child, specifically, is considered the child of the milk-giver and her husband." Accordingly, your nephew's son may marry your daughter, provided there is no other reason for prohibition between them, because the suckling of your nephew's son's sisters by your mother does not make him a brother to your daughter. It is a condition for suckling to be prohibitory that there be five full, satisfying sucklings.