So Charlie's family is awesome, and I have a great relationship with FMIL, but there are definitely weird grudges in that family, and she is the ringleader for a lot of them, especially in regards to FFIL's family.
The other day we went out for a lovely day of shopping for the wedding, had a wonderful time picking out vases and craft projects and general doodads. At one point, I innocently brought up the guest list, and asked her if she knew whether three college-aged cousins on FFIL's side were seeing anyone, because I wanted to know if I should budget for guests for them. FMIL immediately goes from pleasant craft maven to mildly ragey, letting me know in no uncertain terms that unless these cousins got engaged before the wedding, they were not going to be allowed dates, all because back before she was married to FFIL, his family was really unwelcome to her and refused to invite her to family weddings even though she was engaged to FFIL. So basically she wants to pay back a snub from more than 30 years ago by potentially snubbing these kids. And she told me that if I gave them guests on the invitation, she would call and say it was a mistake.
Now, I know this sounds bad, and I was really taken aback, and also with all my years hanging around the E board I know that Emily Post would want me to be a paragon of proper etiquette. But I'm thinking of letting this one go, because 1) in my almost five years of dating FI, I have never met any of these cousins, and I've only heard rotten things about how they used to cheat FI and his brothers playing games when they were kids, 2) I think my relationship with FMIL is more important than these random cousins, 3) FI agrees with FMIL, 4) they're not my family, and 5) we are pretty much at capacity on our guest list anyway. Also, I don't know whether these cousins even have SOs, and FMIL won't tell me.
Is that terrible? Or would you back away slowly from the kettle o'crazy too?

Our Story MAJORLY UPDATED 8/6/09
Wouldn't it be nice to live together in the kind of world where we belong?