OP, if she's being that open about it, she may have her husband's consent and the understanding of his family. I know it sounds weird, but it does happen. So tread carefully.

OP, Bring it up with her and clear the air. But make sure you know in your own mind what exactly it is you want from her. Things are not going to go back to the way they used to be, as you say and one of your posts. If you want to and to be honest and open about her affair, then let her know. If you want her to stop having the affair and stop lying to your uncle, then that's not realistic.