After reviewing FMIL's guest list, I realized that one of his cousin's SO was not on her list. I don't think this is an oversight on her part. FI's cousin's SO is like 5 or 6 years older than him, which isn't that big of a deal, until you find out that they started dating when he was 16. She was working for the kid's mom at the time and volunteered to tutor him in Spanish and the rest is history. I'm pretty sure they're living together now. We haven't seen this cousin in 2 years. When we last saw him, it was at a wedding and he was visibly and vocally upset that his SO had not been invited.
Personally, I've never met the girl. FI is still disgusted by it, but I know if I told him we have to invite her, he'd be OK with it. I really don't see either situation being ideal- cousin comes alone and is crabby or cousin comes with SO and creates an awkward family situation. So, my question is, should I ask FMIL about it and tell her that we have to invite the SO whether we like her or not? Or if she doesn't want her there, can I not invite her?