Answer #3... First let me say.. (2 Thumbs up for JayR & OronD) 2 of my favorite guys on yedda!
I have 2 Sisters, 3 of us all together, and we all have such different personalities, but we always agree that our bond will never be broken, especially by a man in our lives.
Your Twin does sound very difficult for you to deal with. I was always lead to believe that Twins were very similar, so much so that they could finish eachothers sentences! Yours is a very different story.
To me, it sounds like your partner cares for you very much and doesn't appreciate all the trouble your sister brings you. He wants you to be peaceful. So I would suggest being quiet about your personal frustration and concern for your sister around him... stop complaining about her to him, and let her be herself. Don't worry so much about the way she wants to live her life, or how she wants to present herself. It maybe that she acts so terrible towards you and your partner because you act so terrible towards her. You and your sister are two seperate individuals, and even though you are twins you're obviously not the same. When you're with her, be good to her, no matter how she's acting, show her love and give her room to be herself. You're 32 years old now, why do you still want to see her as some sort of aweful reflection of you, or thorn in your side? I say, don't think bad thoughts of her anymore, no matter how ugly she acts toward you, don't flash your partner looks of discust when she's not looking, or gossip behind her back... Just stop! If you can do this, you might find that both of them settle down too.
OronD talked about a 3rd party to bridge between you... the way I see it, you have to be the bridge.