But can't do so with this one. My heart breaks for you, my friend; you've probably read my rants about a similar situation w/my "sister". I use quotes because she's always called me the changeling. It's hard and it hurts like hell, but there are times when you have to turn mother's picture to the wall and get out. Family are the people who care about you, no shared DNA required. If someone proves over a period of time that they take pleasure in hurting you, it's okay to say a prayer for that person and walk out for good. Would you continue to tolerate a man who abused you in any way? Of course not. You don't owe your sister a handy target for her abusiveness either. An accident of birth made you kin; she's broken that deepest of ties too many times for it to hold any sway over you.
Do what I did the day my divorce was final - I sent my ex a greeting card 'congratulations - to me! Free at last, free at last! Thank God Almighty, I'm free at last!"
And don't look back. Declare your freedom and make it stick. This is easier done when you have forgiven someone. It's not to hurt them back necessarily. But it does them no favor to serve as their enabler, either. For years it can be agape love, but even that doesn't require painting a target on your forehead and handing the abuser a pistol. At this point she's not going to change. So you must. Not only for her sake, but for yours and the sake of all who do love you.