Is it normal to think like this

I have observed from last 2 years that whenever my nephew calls us at home, he talks more than an hour with my wife. She usually talks in front of me to him and they are normal family chat only. On all those occasions when she is having chat with my nephew, we have a heavy session in bed at night, mainly she dominates me. I am 46 at present and my wife is 3 years younger to me. We have 2 children, elder son is working in neighboring city and the younger is graduating in our town and staying with us. Even though I am 46 my desire for my wife has not reduced but her energy has reduced a lot from what she was having in her early days. Now most of the time she will give some weird reasons to not have in night. But yes whenever my nephew will call us she will be fully aroused and she will even force me to have with her even if I do not have mood. Same thing has happen yesterday night also. He calls usually once in a month or 2 months. He stays abroad and never been to our house in last 6 years. 6 years back when I was met with an accident and hospitalized for 3 months, he stayed at our house for 15 days to help my family.
From all this, I have weird thoughts that they have affair or my wife is fantasizing for him. I doubt that they have affair as my nephew is much younger to her, nearly 9 years younger and also he has never came to our house from last 6 years but I cannot rule out that she is not fantasizing for him and not for me.