Aack! My brother and I are having issues right now and they are not that far off from yours with your sister. In fact your comment that " I don't really like who my sister has turned out to be" really hit home for me. I say the same thing about my brother.
All that being said, here is what I have decided to do. I do love my brother and up until 2 years ago when he got married, he was great. Rough on a few edges but great none the less. My relationship with him has really deteriorated due to his new found personality. I had to decide if I wanted him in my life or not. I really would rather not deal with him but I also hate the idea that there could be tension at family events, and anger over something that could be controlled. I decided to just make nice when I am with him - period! When I see him I just let it be, I smile ask questions and truly try to enjoy my time with him. Is it easy? NO! Is it better than a big uncomfortable mess in my life? YES!
Here is what it comes down to in my life in bullet point form:
- My mother will be hurt if he and I do not get along and that would crush me
- My children will learn quickly how to be the bigger person
- I only see him maybe once every other month if that - easy
- If there is an event or something we need to be together at, there is usually a lot of other people around. My time with him directly is probably 30 minutes total - easy
- I focus on the fact that the problem is his not mine.
- I know that even though he is a "his way or no way" guy, I still have the choice to say "NO" ! Just because I am being nice doesn't mean he needs to walk all over me!
- I will always show up to his kid's stuff when I can (and I do expect the same from him). It is about the kids not me or him.
- I will always put on my happy face, if I cannot, then I don't feel good and have to go home. = )
In short, don't make your sister's problem your problem. Don't teach your children to be hateful. Go to the dumb party, put on a happy face and let it be. Afterwards, go home and smile that you were the bigger person, and move on.
Good luck!
P.S. You are not being petty, all your feelings are very valid! You are the only one that can control your life so do it.