Our wedding is quite a way off and right now we're more in the brainstorming phases of things than the actually planning stages. However, one thing that comes up repeatedly is the FI does not want his mother to attend the wedding, or know anything about it, including date, time or location. This is because FMIL is a terrible person (and that is the understatement of the century!). She has been verbally abuse to all three of her children through the years, and when he was in his early teens (12,13,14), she got in this habit of actually kicking him in the face when she was upset about something. She also more recently has done things like threaten to find our apartment and burn it down and break all the windows... because she wasn't invited to FI's birthday dinner. So it seems like a no-brainer to not invite her, correct?
The one thing I'm concerned about is that she is still technically married to FFIL. They "live" together, even though she is cheating on him and spends most of her days and nights with one of her three boyfriends. FFIL knows she is cheating, and doesn't care much, but has also made no move to divorce her or separate. He claims he hates her, but still shares a room with her when she's home.
I am totally aware of the etiquette behind inviting all couples with their SO, but in this case do we need to invite FMIL if we invite FFIL?