I think you need to look at the situation here and figure out why you're OK inviting the person who cheated but not the person with whom he cheated.
Regarding the situation IMO you need to either invite both or neither. Consider that at least for the short term, she's your sister in-law after the wedding and decide which situation is worse: inviting her and everyone in the friend group needs to behave and people are seated on opposite sides of the room from each other with a plan in place for trouble. Or you don't invite either of them and that's going to be drama up to and after the wedding for you.
My fiance and I don't want to invite his sister in law to our wedding. His sister in law use to be married to one of my fiance friend which was also his brother's friend. His sister in law had an affair while still married with my fiance brother. Yes you heard right my fiance brother hooked up with his so called best friend wife. You could imagine the drama there was and of course my fiance lost all his friends due to the fact that he did what he did.... my fiance is still friends with the guy along with with other friends and they are all my friends as well ! With her being there we feel that there is going to be some drama/tension. We know for a fact that she would approach her ex husband and the group of friends and act like nothing happened. Which is the reason why we don't want to invite her cause we don't want drama at our wedding and have to feel that we have to look over our shoulders to make sure nothing is happening... me and her don't have the greatest relationship. When my fiance and I started dating I was always nice to her said hi , and all of a sudden she started talking crap about me not only to my best friend but to other people for no exact reason. It's been 3 years no apology nothing. My fiance brothers knows that she talked crap about me he also did nothing but expects us to talk to her ! Cause he doesn't think what she did was bad. My future parents in laws were on my same page when we told them that we weren't going to invite her and understood the reason BUT now when we addressed it again now they are being defensive. His mom made a comment that really got me saying that my fiance is choosing friends over family. So we are suppose to feel sorry for his brother and wife ?? They were the ones that caused all these problems. My fiance and I are NOT uninviting our friends because of them.
So if I’m understanding this correctly, you’re holding her accountable but not the brother?
This all seems immature, honestly. She’s married to your fiancé’s brother. You can’t invite him and not his wife. How are you going to handle any forthcoming family events? Holidays? Why not just be the bigger person?
Re: Wedding
Regarding the situation IMO you need to either invite both or neither. Consider that at least for the short term, she's your sister in-law after the wedding and decide which situation is worse: inviting her and everyone in the friend group needs to behave and people are seated on opposite sides of the room from each other with a plan in place for trouble. Or you don't invite either of them and that's going to be drama up to and after the wedding for you.