Friends can I have your advice?

I really need it. I'm not sure what to do about this. This is the most serious question I have ever asked in almost four years on Y!A. Here it is. Yesterday I stopped off at Denny's to get breakfast. I usually don't go to that one but I was in that neighborhood so I went there. I was sitting at the counter when my niece's husband came up to pay his bill. He was with a young woman, very young woman. He got a really surprised look on his face when he saw me sitting there. After paying he came over and said hello and introduced the girl as someone he met in college, even though she appears to be several years younger than him. I do no think she was looking at him with eyes of just a friend. It was an awkward moment. I just had Thanksgiving Dinner at their house. I thought at the time he was acting very subdued, but I just shrugged it off. Now I don't know. I love my niece. She is my favorite. She has always been special to me ever since she was a baby. They have three wonderful boys. Could he really be that stupid, to risk everything for a fling? What do you think I should do? Keep quiet and mind my own business? What if it gets out that I saw them and it turns out he is fooling around? Will my niece hate me for not telling her? I am trying to think of ways of giving him the benefit of the doubt. It's just that up until they saw me they didn't seem like just two friends who met for breakfast. I hate this more than anything in the world. Help me out here. I'm sick at my stomach over this. I hope and pray I'm just being overly suspicious.