One who is breastfed by your mother becomes your sister and the sister of all your brothers

Question: I have a cousin whom I wish to marry, but it has become clear to me that she was breastfed by my brother—who is younger than the brother next to me—and she was breastfed more than five times, as was he. What is the ruling of the religion in this matter? Is she permissible for me or not?

Answer: If the aforementioned girl was breastfed by your mother five times or more, while she was within the two-year period, then she becomes your sister and the sister of all your brothers from your father and mother—if her breastfeeding occurred while your mother was with your father. If she was under another husband other than your father, then she becomes your sister from your mother through breastfeeding, and the sister of all her children from all her husbands; due to the saying of Allah, the Exalted, in explaining the prohibited relatives in Surat al-Nisa: [al-Nisa:23], and due to the saying of the Prophet (peace and blessings be upon him): It is agreed upon that it is authentic.