Ruling on marrying the daughter of a paternal cousin who was breastfed by his father's wife

Question:
My paternal uncle married two women; one gave birth to a son, and the other gave birth to a daughter. I was breastfed by the mother of the daughter. Is it permissible for me to marry the daughter of the son born to the woman from whom I was not breastfed?

Answer:
If you were breastfed by the mother of the daughter, you become the son of that man, provided the breastfeeding was complete, meaning five times or more. You become his son and a brother to all his children, and an uncle to their children. It is not permissible for you. If you were breastfed by one of his wives, or any of his wives, you become his son, provided the breastfeeding was five times or more while you were within the two-year period. You become his son, an uncle to all his children from all his wives, and an uncle to his daughters as well.

Question: .. : Since he was not breastfed by her, does he not become her son?

Answer: But he was breastfed. If he had not been breastfed, there would be no issue.

Question: He asks: Is it permissible for me to marry the daughter of the son born to the woman from whom I was not breastfed?

Answer: He would be marrying his sister's daughter! She becomes his sister's daughter. No, she becomes your sister's daughter, because you are a brother to all his children. Therefore, the daughters of his sons are your sister's daughters, and you are their uncle, so you must not take her.