No you shouldn't be upset, you should be relieved. Holidays are for relaxing and giving thanks for those you have around you. Start a new tradition. You and your family have a wonderful Thanksgiving AS A FAMILY all on your lonesome. Make a smaller turkey, or have ham, or whatever YOU want free from the expectations or traditions of others. Enjoy yourselves free from the craziness and expectations of others. Does that mean be bitter or nasty about things to the MIL....No! When your MIL calls for help, just tell her "sorry I can't we are planning something for the family here at home". Plus I think we all know someone who has no place to go on the holidays that we can invite to share the day with us. There are a lot of lonely people during the holidays.
From what I gather from your post your SIL is very selfish and self centered and INSECURE, that is why she always has to have things her way or the stuff hits the fan. It's a power trip on her part. This Thanksgiving let someone else deal with that mess and enjoy being free of it. Sometimes the best way to keep the peace is to not be part of the chaos. I would consider letting your MIL know (through your husband of course) that always giving in to the SIL's whims does not promote peace or harmony in any family situation, it actually creates resentment and hard feelings, and that you're both ready to accept your SIL as she is but that you've made the choice not to be party to her tantrums, or be involved in the chaos she creates, so dear Mom/MIL you are on your own here. Tough love works quite well.
So don't be sad, or hurt, just have your own shindig with whomever you choose to invite and enjoy your worry/chaos free Thanksgiving, in your own home, with your own traditions. And maybe next year invite everyone to your house for Thanksgiving, then it's your turf, and your rules and your ways. If the SIL and family chooses not to attend consider it a blessing.
So don't worry be happy. Have a very Happy Thanksgiving.