sister has inappropriate behavior

So my whole family is aware of this, but do not put her on meds since they think it will make her worse. The way she acts in public is very inappropriate and it's usually all the time. She got into an argument with our step dad once and threw a sharpened pencil at his eye since he didn't agree with her on something. She did this a lot, not just to use, but to people in general. I understand why people would be upset over someone having a different opinion than you at times, but to attack them is wrong and that's what she does.
She has threatened to murder me once and would threaten me with knives and scissors and I remember not feeling safe in my own home for about a year or two. Today, I guess I said something wrong and she hit me in the chest (since she knew it was sensitive) and tried "cunt punching" me all at school since she thought she had the right to since of what I said. 
She has gotten therapists, but nothing has worked at all. She is a year younger than me (15, she's going to be 16 in 2017) and she has an actual mental problem she got from our biological dad, but we still don't give her meds since they keep thinking it will make her worse. My mom has a busy work schedule and she works non-stop, so she doesn't have time to really do much with us, and my sister wanted to get her HIV shot (or what it's called), but my mom said she didn't have the time since she had to work soon, so she started having a tantrum in the car. Yelling, telling our mother to kill herself, yelling at her how she's a horrible mother and she should feel ashamed of herself, and she's just kicking and punching the inside of the car. 
I love her and have had fun times with her, but at the same time, I don't want to be around her. I'm only 16, so I can't move out and I don't want to run away, but I don't want to be around her. She encouraged me to kill myself in middle school and I almost did. She verbally lashes out at our mom sometimes, and I'm too scared to step in since I know she will hurt me. I just wish we could do something about her, but my parents won't. What do I do? What would you do in this situation?