Bring her at the kitchen table and have the brother next to her. Tell her that today is a new day and that she is not allowed to be nasty verbally or physically anymore. She is old enough to control herself and if not, she'd better learn quickly because otherwise she will be left with nothing, as for every scream, you remove forever something she likes, her favourite dress, her bike, favourite doll, for every physical attack, no matter if a tiny scratch out a hard kick, TWO things are removed. Every time, she will be send to her room and you must enforce it. Her behaviour is unacceptable, she will be punished for it and send away,
I would be careful before I tried this approach. The two children in my family who displayed similar behaviour both had some MH issues though in one case it wasn't properly diagnosed until years later. In both cases, their behaviour was connected to deep underlying anxiety. A gradual removal of possessions would simply have meant that eventually they had no toys or clothes left and that the behaviour would have escalated. Removing their comfort toy would have sent them completely over the edge.
At the same time, there was the obvious issue of needing to protect siblings- particularly for my dd whose brother was 3 years younger.
In the other instance, I was one of the siblings, so have an idea of the impact on us.
First of all, we knew exactly what behaviour was expected of us: we did not have to accept unfairness, we were not held responsible for db's meltdowns, we did not have to spend our lives on tiptoes.
But we did have to realise that there were certain situations he couldn't cope with and that our parents would have little sympathy with anyone who deliberately put him in those situations for the fun of it. An older child deliberately teasing or winding a younger child up was unacceptable and we knew it.
At the same time, we also knew that if a meltdown happened, our parents would intervene immediately to keep us safe. We would have to remove ourselves from the immediate target area, but they would not allow us to get hurt. Db would be restrained rather than allowed to hurt anyone. We had locks on our doors and were allowed to use them.
It also helped that fighting was not allowed at all in our household, so there were no grey areas. Any physical violence would be broken up immediately, and the offenders sent off to separate rooms.
It is not quite clear, OP, whether your dd has actual meltdowns- i.e. times when she is totally out of control and hardly knows what she is doing, or if her behaviour is more deliberate. If it is the latter, then you can punish for it, though I would be wary of the kind of punishment that spirals out of control. You can also combine punishment with positive reinforcement, praising her for when she finds a better way of dealing with anger.
If it is a proper meltdown, then I wouldn't punish or even mention it much in retrospect. A child like this will be terrified of the violence within them and will need the reassurance that this isn't really who they are. They may not even remember clearly what it is they've done: I'm not sure my db did, but I do know from dd (who speaks much more openly about her inner life) that she lost touch with reality at some point.