Ruling on marrying the sister of one's sister through breastfeeding

Question:
The last question from our brother says: I have a sister from my mother who was breastfed along with my maternal uncle's son. My maternal uncle's son proposed to marry my full sister. Is he permitted to marry her or not?

Answer:
If the sister from your mother was breastfed by his mother, there is no harm in his marrying your sister—your full sister—because there is no breastfeeding relationship between them.
However, if he was breastfed by your mother along with your sister from your mother, with a complete breastfeeding of five sucklings or more, then he becomes a brother to all of you. It is not permissible for him to marry either your sister from your mother or your full sister, because she is his foster sister.
As for your sister from your mother, if she was breastfed by his mother, she becomes a mahram (a relative one may not marry) to him, and he becomes a mahram to her, provided the breastfeeding was five sucklings or more and occurred within the two-year period. As for your full sister, who was not breastfed by his mother and he was not breastfed by her mother, it is not forbidden for him to marry her. Yes.

The presenter: May Allah reward you with good, O Shaykh.