Well, it's a very difficult situation I must say. First, you have the fact that they recently just got married. I've heard something about people only showing their true colors once they're in a committed relationship.

It might not be up to you. Serious matters are for the people involved to decide. The best you can do is to be honest and supportive of your uncle. If the worst happens and he becomes terribly unhappy, he'll have to make his own decisions. Never assume responsibility of other people's problems. It never ends well.

What I did when I had an unpleasant family member was try to get to know him more. Hear his side and learn more information. If he turns out to be really that bad, then it's out of our hands. We can't change people.

You may start a discussion with your uncle and tell him what you have heard, but do not insult her or say anything bad about his wife because your uncle may hate you for that, you may ruin your relationship with him, just make him understand that you disapprove her behavior.

I can see how hard this will be for both of them. It's like trying to say you don't like mayonnaise to someone who clearly loves it. It's possible to say it in the nicest way. But the effect may not be entirely avoidable. After all, she is his wife. It'll be up to him to recognize the behavior you're seeing.