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.
Yes, it is permissible for a man to marry the widow of his paternal uncle if there are no marriage impediments, like suckling or the like, or if she is his maternal aunt, and the like. The evidence is the saying of Allaah after mentioning the women whom we are not allowed to marry: (interpretation of the meaning): {All others are lawful…}. Imaam Ibn Katheer said regarding the interpretation of the above verse: 'Women other than the Mahrams (non-marriageable woman) mentioned in the verse are lawful to you.' 'Ataa' and others said the same thing.
Allaah knows best.