I have an etiquette situation that I'm not quite sure how to handle. It is VERY important to my FFIL that his close family friend, we'll call her Karen, is invited to the wedding. Karen is also the mother of my fiance's brother's girlfriend. So, although I have never met Karen, I am perfectly fine inviting her. The problem is her husband/live in boyfriend (no one seems to be sure if they are actually married or not), we'll call him Dennis. Dennis is a drug dealer and is physically abusive towards Karen. Karen and Dennis have split up in the past, but are currently together (many friends have tried to get her to get help and leave him, but she chooses to stay). In addition, Dennis and my fiance's brother (who is a groomsman) have gotten into a physical altercation before (when my fiance's brother witnessed Dennis abusing Karen)...so I know it will be uncomfortable for my fiance's brother if Dennis is there.
So, am I obligated to invite this man to my wedding? I know that etiquette says never to split up a couple, but is this an appropriate exception? I don't want to offend anyone, but inviting this man seems like trouble.