While I understand that you do not want to exclude S, it's important to realize a few things.
First, baby steps. You MIL is willing to start mending bridges. Pressuring her to go faster than she's ready to will likely backfire. People tend to react poorly to "all or nothing" ultimatums. She's already shown that she's capable of not talking to her own son over her disapproval of his choices. I wouldn't test her resolve.
Second, it's her house. Frankly, it's not up to you to decide whether or not S should be allowed to come. That also means it's not you who's excluding S, it's your MIL. Don't carry the burden of your MIL's choices.
Third, S is part of your family. That does not make her part of your MIL's family. Sorry, but it just doesn't. My MIL died still hating me. I was never part of "the family." I was "that bitch who stole my son away from me." See, I showed my then-fiance that it was okay to stand up for himself and live his own life. I unwrapped him from around her little finger, and she hated me for that. Sometimes your in-laws welcome you to the family, sometimes they don't. They're ultimately the ones who get to decide who is "part of the family" and who is just "that bitch who married my son."
By the sounds of it, she's not even inviting you for The Thanksgiving Dinner, she's inviting your for a post-thanksgiving dinner. So that means you're free to have The Thanksgiving Dinner with just your husband and partner. Why not start your own traditions? If things go well with you, your husband, and your MIL at thanksgiving, maybe you can invite her to your family's home for Christmas dinner?
Gralson: my husband. Auto: my girlfriend.
Zoffee: Auto's husband. Cue: Zoffee's boyfriend. Bookie: Cue's wife.
Last edited by SchrodingersCat; 11-18-2012 at 09:37 AM.