Ruling on marrying the daughter of one’s wife’s brother through breastfeeding

Question 3: A man wants to marry his wife’s foster niece. What is the ruling on this?

Answer: It is impermissible for him to do so unless he divorces his wife and she finishes her `Iddah (woman’s prescribed waiting period after divorce or widowhood), because it is impermissible to marry a woman and her paternal or maternal aunt at the same time… read more here.