If you aren't close to your other sorority sisters, why bother? It would be one thing if you wanted to see your other friends. Then maybe you could just avoid your mortal enemy during dinner. But since you don't really care all that much about the other sorority members, why put yourself through the drama over Thanksgiving?
Of course, I was never in a Greek system in college so I don't understand the appeal. But it seems like more of an obligation at this point than something fun and meaningful. Plus, with Facebook, you're probably way more in touch with your sorority sisters than you'd really like to be. Why not organize a post-Thanksgiving meet-up or a Christmas party with the ones you really want to see? Or if seeing them isn't a big deal, then maybe your trepidation is a sign that that time of your life is best left in the past. Don't feel like you have to see them. Why bring a negative presence from your past into your life? Remember the good stuff from those days and keep the "mean girls" and drama queens in the past.
Geez. Do what I did and drop. You'll save money, and it's a freeing feeling to not be tied down to sitting through boring meetings with 120 girls you don't even like.