Hi Jammie,
I have just read your thread and am so sorry you are feeling so badly.
As I read your words I was however reminded of the heartache of grandparents on this forum who are not able to see their grandchildren because of a falling out with their children/family feud.
The pain you feel now would, of course, be nothing compared to the pain of not seeing your grandchildren? I just worry that this invitation could possibly be an olive branch and if you are unable to take it, then it might be a while before their is another.
Once you are there, would not the joy of spending time with your grandchildren ease the pain you feel in your SIL's prescence? I have my grandson here with me today, and one hug, one smile from him, makes every worry of mine, every pain fade away. I guess what I am saying is that the giving and receiving of love from your grandchildren is surely much, much greater than the pain/awkwardness of seeing your SIL. No matter how unjust he was, how horrible it was, how uncomfortable a meeting may be, can you not just focus on the grandchildren who you love so much. Nothing will ever be more important to me than seeing my grandchildren.
Good luck whatever you decide.