Ruling on marrying the sister of a brother through breastfeeding

This is a message we received from Oman (S. B.), the Hashemite Kingdom of Jordan, from Abdullah al-Fayy al-Basit, via the Call to Islam Radio from Makkah al-Mukarramah, on the program "Light on the Path." We ask you to kindly answer the following two questions, and we thank you greatly. In his first question, he says: I have a brother who was breastfed by a woman, and she has a daughter. Is it permissible for me to propose to marry this girl, or not?
Answer: If your brother was breastfed by her mother and you were not breastfed by her mother, there is no harm. It is only forbidden for your brother. If he was breastfed by her mother, it is forbidden for him, provided the breastfeeding was five times or more within the two-year period. As for you, you have no such relationship, so there is no blame on you in marrying her, if she is a stranger whom your brother was breastfed by. If the breastfeeding was complete—five times or more—and it occurred within the two-year period, then she becomes his mother, and her daughters become his sisters. As for you, you are a stranger, so there is no blame on you in marrying one of her daughters. Yes.