I'm in love with my wife's sister
What caused you to end up in love with your wife's sister?
I think the beer at Christmas is what causes this to be honest.
It is very wrong to be in love with your wife's sister. You should bury your feelings so you don't destroy your family.
Maybe, I'm excited
This would break your wife's heart to find out that you are in love with her sister. However, it may be a sign that you should not be married with the person that you are married too. Just think this would be something difficult to keep a secret for the rest of your marriage.