Ruling on marrying the daughters of a maternal aunt through breastfeeding

Question: I inform Your Eminence that my mother was married to a man before my father, and she bore him a son. She also breastfed her sister along with this son, and the breastfeeding lasted about a week. Then my mother separated from this man, and my father took her. Is it permissible for us, the sons of the second husband, to marry the daughters of our maternal aunt who was breastfed by our mother, or not?

Answer: It is not permissible for you to marry the daughters of the aforementioned maternal aunt; because through the aforementioned breastfeeding, she became your sister.