Prohibition through Breastfeeding

Question:
We ask for an explanation of the breastfeeding that creates a prohibition, since some people marry the sister of their younger brother who is related to them through breastfeeding. Is this permissible?

Answer:
Breastfeeding has been clarified by our Prophet (peace and blessings be upon him), and the Qur'an made it clear. Allah (Exalted and Majestic is He) said: [Surat al-Nisa: 23].
So the prohibition through breastfeeding is found in the Noble Qur'an regarding mothers and sisters. Then the Sunnah came with the prohibition of the rest, just as in blood relations. The Prophet (peace and blessings be upon him) said: When it was said to him, "Will you not marry the daughter of Hamzah?" he said: .
And when the maternal uncle of A'ishah through breastfeeding asked for permission to visit her, the Prophet (peace and blessings be upon him) said to her: This indicates that breastfeeding is like blood relations.
So if breastfeeding is established by the legal evidence... a trustworthy woman testifies that she breastfed the woman five times, and the child was within the two-year period; then the ruling of breastfeeding is established for five times or more, and the child is within the two-year period. In that case, breastfeeding is established, and the woman who breastfed becomes his mother, her children become his siblings, her father becomes his grandfather, and her mother becomes his grandmother. This is what follows from blood relations, and it also follows from breastfeeding.
But some people may make a mistake in these matters. If his younger brother was breastfed by a woman, and the older brother was not breastfed by her; then he may marry the sister of his younger brother; because he was not breastfed by her mother, only his younger brother was breastfed... by his mother.
So the conclusion is: if his younger brother was breastfed by a woman; her daughters become siblings to the younger brother, and it is not prohibited for his older brother who was not breastfed. If Zayd (a younger brother) was breastfed by Zaynab, and he has older brothers, and Zaynab has daughters; then they are permissible for the brothers of the breastfed child, but they are not permissible for the breastfed child. The breastfed child is their brother, but his brothers are not prohibited for them, and they are not sisters to his older brothers; because they were not breastfed by their mother, and they were not breastfed by the mothers of the older brothers, nor by their fathers' wives, nor by their sisters, so they are not prohibited for him, even though they are prohibited for his younger brother who was breastfed by their mother.