Let me start by saying I normally have a very good relationship with my FMIL. She's a sweet, giving, and generous woman. Her husband passed away a few years ago and her only child, my FI, is her whole world. She's been very excited about the wedding, and for the most part has not made many demands that we couldn't meet.
Except today she done gone crazy.
We're having a small 100 person wedding, so ten tables of 9-12 people. FMIL wants to shareTable One with my parents and a few of their siblings (fine), then she gave me a specific list of who her side should be grouped with, which we accommodated as best we could when combined with who on my side can and can't sit together, and she wants all the tables around her to be her friends and family, which isn't possible
I had FI call her and explain that she can either A) sit at Table Five and have more friends around her

host her own table separate from my parents so she can have more of her friends and family around her or C) just accept that the tables aren't going to be to her exact specifications, but everything will be fine.
If we did what she asked, the bridal party/best man would be 30 feet away from us when giving their speeches and my side would be uncomfortably mixed with divorces couples and people with VERY different political viewpoints sitting together. I would say they can all be adults and behave, but we've had some uh, boisterous conversations at holiday gatherings so we chose to separate some people. I assume it is the same with her side, so we did the best we could to make everything work out on both ends as was possible. I'm sure these are the exact same things everyone deals with when assigning tables.
After being on the phone with FI for 20 minutes while he explained the situation to her all I have is a FI that feels annoyed and slightly guilty and an email from FMIL about how important this seating is to her and that is it "protocol" to have the families near the parents of the bride and groom and this is going to basically tear her family apart. Also how I should have dealt with her directly instead of getting FI involved and having him upset with her now. None of FI's logic or explanations seemed to have sunk in at all.
A big factor is TWENTY FIVE YEARS AGO when her brother got married she was seated at a random table with cousins instead of in the front with her siblings. She is still seething mad about this and has brought it up at least twice a month for the five years I've been daing FI. She's terrified of her brother's wife, and even though she doesn't like her it is important to her to impress her by treating them the opposite way of how she feels she was treated. I'm really not interested in having my wedding be the grounds on which to settle this silly feud, especially if it costs us an extra $400 to have the extra tables we would need.
The wedding is in a little over a week, and tensions are running high. I would love a little advice on how to smooth things over. It isn't feasible to accommodate all her demands although we did the best we could. I did give my mom a heads up since they're close, so hopefully a little MOB & MOG talk will help. I just really don't want this to affect our relationship permanently. Its the first time FI and I have teamed up and clashed with her over something "serious" and I think she's just stressed about the wedding and possibly feeling she's "losing" her son. He's irritated with her for being so dramatic and isn't helping the situation.
I'm going to let her cool off a little and send the updated seating chart later today and CC my mom and FI in, but I don't want another explosion over it.
Now I'm cranky and stressed at work trying to figure out how to fix this.
CN: FMIL has unrealistic expectations of the seating plan and is upset with me for having FI talk to her about it.