Is it permissible for a boy who was nursed with another boy to marry the sister or niece of that other boy?

If an infant nurses from the mother of a second child five times or more within the two years (of breastfeeding), then it is not permissible for him to marry the sister of the child he nursed with, nor the daughter of that child's sister from that woman. This is because the first is his sister through nursing, and the second is his sister's daughter, based on the hadith: "What is prohibited by lineage is also prohibited by suckling," and Allah's Almighty saying: {And your sisters through suckling} (An-Nisa 4:23). However, if he nursed with the second child from another woman, there is no objection to marrying that child's sister or her daughter, unless there is another reason for prohibition.