It sounds like you both have some growing up to do, really. Wow. This is a really unfortunate circumstance. My own sister and I had never had it out in the way you describe. She was a hitter, but her violence was just cute and funny, and never out of control. I'd just be laughing.
This just sounds.. Dysfunctional, on both sides.
I'd suggest compromising when it comes to decision making. Sometimes you've got to do the unnecessary things. And maybe spend some time together sharing interests. For my sister and I, we liked playing video games together and watching television. See, if you're both stubborn and nobody's willing to compromise, you can't get along. So if she won't, then you have to. Sometimes leading by example is the best way to show a method that works.
If this ends with you bending over backwards and being a door mat, though, then obviously you should take a stand.
Try to understand things from her point of view-- Why is she acting the way she is? It might seem completely without logic or rationale, but everyone has a motive. Something that drives them. Try to understand your sister better, and maybe you can make this work out.
Don't depend on her to change, and don't make her depend on you to grow up or to stay in line and be an adult. She has to be allowed to make her own mistakes, however terrible those mistakes can be. You can't always be there to reign her in. Making mistakes is how we learn, and sometimes we have to do a lot of learning.
You're not responsible for the way she's acting, she is. You're only responsible for your own thoughts, feelings, and reactions.
That's my take on this.