So my Mom, against my wishes, emailed her sister to tell her that she and her family are not invited. I was for the "they'll figure it out when they don't get an invitation" approach, and asked her not to do anything. Oh well... she sent her this email (edited for anonymity):
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Hi (Sister),
I hope all is well with the Smith Family. It is certainly nice that the weather has been so good. I wanted to let you know that Calindi and Trip have finalized their guest list for the wedding. A truly daunting task. I thought with multiple families our side would be the problematic one but it seems Trip's has me beat with nine aunts and uncles, their spouses, and 70 first cousins. Calindi has made the decision that she absolutely wants her grandmother at her wedding. It proved to be too much for Mom in the recent past to attend either Calindi’s or (Calindi's brother)’s graduation parties because you and your family were going to attend. Therefore, Calindi has decided it would be best to not invite the Smith family to her wedding. I hope you understand, this puts me at quite a bad place, but I must follow my daughter’s wishes. I will miss sharing this day with you, but it is what it is. Love (Mom).
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Way to throw me under the bus, Mom! I'm not too mad about it, because I know this does indeed put my Mom in a bad place even though she isn't particularly fond of her sister. I don't think she's seen or spoken to her in 2+ years since my brother's graduation party (which as mentioned above, Sister and family attended, while Grandma did not).
However, sister wrote back a very nice email:
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(Mom) and Calindi,
While it breaks my heart that the dysfunction of our family lingers, I completely understand your decision. Calindi's wedding day should be filled with joy and free from tension. (This spoken from a woman who got married in a swamp due to its secret location which no one could find! It was actually quite a lovely swamp. 
) Please don't spend a moment more worrying about your decision. Know that my prayers are with you for a beautiful day and a life long marriage filled with love.
Love, (Sister)
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So the biggest guest list drama seems to be behind us! Now we'll see how my Mom's Dad reacts, who is the one who has been causing all the tension about this issue since he found out back in December. Oh well, what's a wedding without family drama, eh?
Next up - getting FMIL and FI to give me emails and/or phone numbers of everyone on their guest list. FI is finishing up finals, and then will be lost to the world of bar studying, meanwhile FMIL is neck-deep in accounting pending the April 15th deadline but is pretty much no help as about 1/4 of the addresses she gave me for save the dates were wrong... but we didn't get the returned envelopes until nearly 2 months later! FI says if we send invitations and don't get them back nor an RSVP back, we should just assume they're coming. Aw, helllll no! So I'm trying to prep all the phone numbers now, as when that happens will be literally right when he's taking the bar exam so he'll be no help at all. I forsee a headache!