All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have listed all the non-marriageable women in Fatwa 85868.
Marrying any woman other than those who are mentioned in the above Fatwa is allowed. So, it is allowed for a woman to get married to the husband of her paternal or maternal aunt after the death of the aunt. It is also allowed for a man to marry the mother in law of his son or daughter if there is no blood or suckling relation that forbids marriage in either cases.
Allaah Knows best.