I am scared of my little brother

I (F) am scared of my younger brother (16M).
My younger brother is adopted which I include only because I believe it is relevant to this. He was adopted very young but still spent a couple years with his bio parents but was adopted young.

He is very violent, impulsive, secretive, etc and has been since he was very young. As a kid he tried to drown our cat once and shot another cat with a pop gun toy. My parents tried to socialize him before school through daycare but everyday was a list of bad things he did that day (one of the most notable being that he broke a little girl’s collarbone).

He lied about literally everything as a kid and didn’t care what hurt it caused. When my brother started middle school he was destructive and violently attacked another kid at which point we removed him from school and enrolled him in a homeschool program on a device we had attempted to lock so he could only access that.

My brother also has a huge fixation on weapons so we don’t allow any around him but he always find a way. He is obsessed to a worrying degree. worrying degree. For example, my mom left to go to the cemetery a few years back and my brother snuck out to the shed and stole a knife from my dad’s tackle box and hid it under his mattress. My dad found it after we kept having money go missing. It turns out he was also stealing our prescription meds. None are even meds someone could really get high off of, he just took them because he could. We added reinforced locks to all doors in the house so he couldnt take things from us. One point I was in the hospital and hadn’t locked it and he stole money from me.

He continuously did bad things to us and we were unable to find a psychiatric facility to take him so we enrolled him in a very good military school program because we had heard positive things from people who went there. (It basically has stuff you would expect in a military school like early wake ups and routine and stuff but with an added bonus of fun trips and things). While there he made a weapon and was caught with it .
My dad took him to a hospital for psychiatric hold. They found a place finally to take him but the place had to hire extra male staff to work overnight because my brother repeatedly attacked female staff. Eventually the place sent him home and we were told there’s really nothing that can he done until he kills someone. He also admits to abusing our dogs.

I was home alone with him one night a few months back while my parents were with friends at a concert and they don’t get to go out often. I checked on him and then showered and came to check on him again at which point he tells me he drank some nyquil and four cups of coffee which is strange because he usually doesn’t tell what he does and tries to hide it. He had punched a hole in his wall and told me it was because he missed our dad who he doesn’t like and is later ranting about how much he hates him. I usually don’t point out when he does stuff like that because it just makes him mad but I did for some reason. I think he doesn’t like when manipulation doesn’t really work. And he got this really scary look in his eyes that I can’t explain and started shaking very violently and punched the couch beside my head really hard. He is taller than me and a lot stronger than me and I was scared. I can’t explain it but I just had a feeling in that moment that he was gonna kill me. I ran outside and called my older brother and SIL who don’t live far and they came over. Eventually we go in and my older brother points out the same flaw in his logic and my younger brother gets in a physical fight with him where my younger brother is trying to attack him while my older brother is trying to get him contained in his room.

My parents come home and my younger brother gets that look in his eye again and tries to attack our dad. Me, my mom, my older brother, and my SIL are all trying to hold him back but can’t. He says he’s going to kill our dad. We manage to hold him down finally and call 911 and he is on this rant about how he oughta be able to do what he wants and how there’s nothing wrong with him. The cops come and take him in an ambulance and tell us the same thing people keep telling us that nobody will really do anything.
We tried really hard to find another place to send him and everyone kept saying there’s nothing unless he is in state custody because facilities have to have open slots for kids in state custody so my parents went through that process. He went to another place and attacked multiple kids and was sent to juvie.

He is now is a new facility and advancing through the program despite failing drug tests and still doing things. He sends letters that are basically catered to what he thinks each of us want to hear (I can’t explain it). But he isn’t getting help here just lying his way through. He will be sent home eventually without help and I truly believe he will kill someone. I am afraid of him and feel like a prisoner in my own home. This is just a handful of incidents from a lifetime of interactions like this and I don’t know what to do. Does anyone have any advice or has anyone been in a similar situation? Sorry if I left anything out I will reply to any questions in comments.