The ruling on a man marrying his sister's sister through breastfeeding

The questioner:
Another man asks, saying: I have a sister from both my mother and father, and my maternal uncle has a daughter and a son. His daughter was breastfed by my mother, and now my maternal uncle's son…

The Shaykh:
Hmm.

The questioner:
Seven. A man has a sister from both his mother and father, and his maternal uncle has a daughter and a son, so his daughter was breastfed by my mother,

The Shaykh:
He has a daughter outside, but no son.

The questioner:
Ah, so his daughter was breastfed by his mother, and now my maternal uncle's son has asked to marry my sister. Please give us a ruling: is it valid or not?

The Shaykh:
Your maternal uncle's son was not breastfed by your mother, and the one who proposed to her was not breastfed by her mother, nor by her father's wife, nor by her sisters. There is no breastfeeding relationship between them. The fact that his sister was breastfed by your mother does not prevent it. Even if his sister was breastfed by your mother, it does not prevent him from marrying your sister; because he was not breastfed by your mother, and she was not breastfed by your mother, nor by a woman whose daughter is being offered to him. So there is no harm in it, but there is another breastfeeding relationship, yes.