Hi Beam and welcome,
If you could do a signature, it would help a lot. Click on your name in the upper right hand corner and then click on "signature". Just ages of kids, which one is your challenging one, and maybe that you're in Europe would help since I'm sure things are done a bit differently there.
Your son (and you) have the right to be safe in your own home. Age 12 is far too young to have to deal with the raging of an older sibling, in my opinion, especially when it's her stated intention to menace him. That is abuse, plain and simple. It's especially concerning if he's left alone with her - our challenging kids can be real masters at torturing sibs.
It would be good to come up with a safety plan for him. When daughter starts her threatening behavior, son should... go to a friend's house, go to the park, go to a safe room in your home.... whatever works for your situation. My bias would be to make sure he's able to go somewhere there's an adult so that he's not having to manage this by himself. It's not a matter of letting her walk all over him - it's protecting him from her. And he should not be having to defend himself in his own home.
My younger kids learned early to head to my room when things got violent around here. I kept toys/TV/etc in there for them so they weren't being punished for my challenging kid's behavior. But the reality is, his behavior affected them anyway. If I had it to do over again, I would try harder to shield them from his rages.
If items were being thrown or broken, or my kid was threatening to harm me or the siblings, I called 911. The way I looked at it, I wouldn't have tolerated that kind of behavior from a spouse in my own home - I sure as heck wasn't going to tolerate it from a kid. I remember feeling completely impotent when it came to controlling my challenging kid, but.... sometimes you just have to call in reinforcements.
Sorry your family is going through this. Gentle hugs.