Good for you for remaining strong. I have 2 sisters and the relationships are strained. I used to absorb the drama coming from them and internalize it and make it my own. I don't anymore. Your sister has a charity event. God bless her! You don't "have" to go no matter what kind of guilt is dished out at you. Don't let your buttons be pushed. How has she been supporting you during your recovery? 2 way street my friend.
I was always, always a people pleaser and a giver, but the problem was I resented it. I didn't give from the right place. I now choose and pick what I will do and I "try" not to worry about the other's opinion of me because their opinion is none of my business.
You are on a very important path and I would do whatever you know you need to do to stay on it. If it means not going to the charity event then by all means don't go. Whatever you have to do is for your own well being and that's not selfish, that's life saving for you.
Bless your sister and continue to work on your recovery. That's my opinion on it, but its just an opinion.