The Ruling on Marrying the Wife of the Paternal Uncle or the Maternal Uncle After Divorce or Death
Question :
Is the wife of the paternal uncle (the father's full brother) permissible for his brother's son after she is divorced? And is the wife of the maternal uncle (the mother's full brother) permissible for his sister's son after she is divorced?
Answer:
It is permissible for a man to marry his paternal, or maternal uncle's wife after she is divorced. Similarly, his brother's wife and his brother's son (may marry), if he divorces her and her waiting period is over. Those that are forbidden for him are his son's wife or his grandfather's wife, or his grandson's wife, because she is forbidden to him in perpetuity.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Page 234