I can't always perform sexually with my wife so sometimes I think about her sister when we are having sex.
Why do you think about your sister-in-law?
I know she is a part of the family, but I find her sexy that's why
It seems that you have disconnected from your wife emotionally. You should work to fix the lack of connection you feel with your wife, instead of making things worse by fantasing about someone else.
We are doing fine. I bet she's not a virgin. I don't want to talk about it anymore.
If you have to think about someone else I don't think you are fine. How would you like it if your wife was thinking of one of your family members when you were in bed together?