Stumped

Hello Everyone:

Well I haven't decided how I'm going to respond to Sis's invitation this weekend.  The main problem is what to say and how to say it without being mean or rude.  I still feel strongly against the whole thing.  I feel like this is nothing more than an obligatory celebration, not something someone wants to do.  I was thinking of emailing an honest response to my sister, essentially telling her "thank you but no on both suggestions" and why.  It's the why part that has me stumped about what to say.  I have this email composed in my head that starts out nice but ends nasty.  Not good.  I could call her but she really doesn't answer the phone much, come to think of it, neither do I.  Mom is politely staying out of it, even though she is part of the problem.  She acknowledges that she really messed up but I'm not happy with her choice of wording, "over looked."  To me, "over looked" means you mixed up days and times or completely spaced out and forgot; now feel remorseful and want to make immediate amends.  "Over looked' seems a little mild.  This was just plain callous and insensitive. If you haven't figured out by now, the emotion is still raw after two-and-half years.  I feel the same way about the abusive ex never making full and proper amends to me-raw.  In his case, I've given up on the matter because too much time has passed and whatever he has to say to me means very little if anything at all.  Besides that, he'll just make up some lame reason per usual.  Of course, I could say yes to one or the other of Sis's suggestions and hope for the best.  Maybe it would be nice.  Maybe, the have recognized how much they hurt me and want to make amends.  I'm not that much of an optimist.  The only thing I promised mom is to let her know one way or the other.