Is it permissible for a brother by suckling – who did not suckle with his elder full-sister, but rather suckled with his younger sister – to marry his elder sister? And why did the Prophet, peace and blessings be upon him, specify five sucklings for the prohibition?

Scholars have differed regarding the amount of suckling that makes marriage unlawful. The preponderant opinion is that fewer than five known sucklings do not make it unlawful. If someone suckles from a woman five known sucklings, she becomes his foster mother, and her children become his foster brothers and sisters, based on the Prophet’s (peace be upon him) saying: "Suckling makes unlawful what lineage makes unlawful." The foster mother's husband is considered the foster father of the suckled child, and it is unlawful for the suckled child to marry his daughters. As for the suckled child's siblings by lineage, they are not considered siblings to his foster siblings. So, if your brother suckled from your mother five sucklings, it is not permissible for him to marry your sister's daughters. If you are the one who suckled from her mother, then there is no harm in his marrying your sister, unless there was another suckling relationship between them, or his sucklings from your mother were fewer than five. As for specifying five sucklings, it is a revelation from Allah the Almighty.