Is it legal to marry the sister-in-law of my father's own brother?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is lawful for you to marry the sister of your uncle's wife. There is no disagreement among scholars over this issue, unless she is forbidden to you because of legal reasons (in the Sharee’ah), such as ties through marriage, breastfeeding, …etc.

For more benefit about the women who are Mahram to a man, please refer to Fatwa 85868.

Allah knows best.