What is the ruling on a brother marrying the sister of the girl from whom his brother was nursed, and what is the ruling on the nursed person marrying the sister of the girl from whom he was nursed?

If someone is nursed by a woman five times or more within two years, he becomes her son, and her children become his siblings. This is based on the Almighty's saying: "And your mothers who have suckled you, and your sisters from suckling" (Quran 4:23), and the Prophet, peace and blessings be upon him, said: "What is unlawful by lineage is unlawful by suckling."

Therefore, it is not permissible for the nursed individual to marry a sister of the woman who nursed him, because she is his maternal aunt through suckling. As for the siblings of the nursed individual who did not share in the suckling, they are not mahram to his maternal aunt's daughter, and it is permissible for one of them to marry her, unless there is another legal impediment.