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.