Anonymous wrote:"He would say she was dressed inappropriately, was talking to boys, was being disrespectful to him, should respect him because he is older, etc. She did wear some tight clothes like skinny jeans and has guy friends but that's it."
I am concerned about everything you have posted here but this particular thing makes me wonder--why is he concerned about this? Why does her attire and relationships with boys set him off? This is yet another red flag for me.
These sound cultural to me--there are definitely places where brothers are routinely very controlling over their sisters, especially about what they wear and how they interact with boys.
OP, you need to remove your sister from this situation. Perhaps she can stay with a friend, but don't tell your brother where, until something can be worked out for your brother. I'm not sure I'd tell your parents; it sounds like with sufficient threats from him they might tell him
He can be committed if he is a danger to himself or others--I am surprised the police didn't suggest that with the earlier attack when they were called in. It is a 72 hour hold, but weekends don't count, so if you get him in on a Friday afternoon, the clock won't start ticking until Monday, and only after the psychiatrist sees him. With luck, that would not be until Tuesday so you will have bought an extra three days.
It sounds like your parents just really aren't ready to admit the gravity of the problem or don't know where to turn. People have offered a lot of good suggestions like calling DCPS etc. But you could arrange a family psych consult with you and your parents and have a professional advise on a next step. They are much better acquainted with all the local resources that are available that could be helpful for you. This would be lower key than bringing in the authorities and possibly could result in a successful plan for convincing your brother he needs help and taking it. The police and other authorities would always be immediate options of this doesn't work.
Finally, am concerned about previous bath salt use and possibility he is doing it again. Bath salts can induce severe psychiatric behaviors.