Bptoo,
Family tension and reunions should be outlawed. My personal opinion.
After reading your post I can understand your hurt and anger towards your sister. It's a tough call. You don't know how she will be or what you should do or say.
I believe in honesty. I don't know that the reuinio would be the best time to hash out the issues but it will give you a semi-safe environment to feel the situation out.
It may well be that now she is seeking pity that is not deserved but at the same time she may have hit her "rock bottom" and this is her way of mending broken relationships. She may have realized what she has done. If that's the case you have to give her credit for facing the family.
Alocoholism as you know is a terrible things which can make people cold, cut off and selfish.
I really wish you the best at the reunion. I know it will be tough but you will have the support of other family there with you. I hope in the end things work out however you would like them to. It sounds to me that you in some way would like for everything to be okay someday between your sister and yourself. That says alot about a person when they are willing to forgive the unforgivable if possible.
Take care!!
Heidu