Bearing in mind that kids are better parent trainers, than we are kid trainers, you might want to modify what you're doing.
Since she's clearly aware of what she's doing I'd suggest that, during a quiet moment, before an outburst, you sit down with her, explain that what she's doing in not OK, and that if she chooses to do it you will . . .. and that she get's one chance, that if she does this more than once in a row she's done playing with the other kids for the afternoon, and she'll be spending the day alone in her room. Also, when you put her into her room, keep your talking to a minumim. Since she'd doing this for attention don't give it to her. If she ask's why, have her tell you what you said during your quiet minute, pointing out that since you'd told her the rules, she has made the choice to go to her room. Put it on her, and then leave the room. On that note, clearly, what you're doing is not working. She must be held accountable. I'd suggest that she do more than verbally apologize. At that age, it's ususally meaningless. She must give the other person one of her "special" things. If it's an adult or older child have them choose it. If it's a baby you do it. Also, I would not let her go right back out and play. After she's done screaming I'd go in, tell her how sad and upset her behavior makes you, and tell her that she must draw an apology picture etc. and that when it's done you will consider letting her play again, but that she must earn that right.
Also, when you put her in her room don't let her play with her toys. In our house, in your room means you're sitting on your bed. You might want to consider a time out chair facing a corner. Trust me, it's really boring to them.
I would also help her verbalize her anger. When she say's she's going to hit someone sat somthing like "wow, you must be really angry, what's up?" Let her know that it's OK to be angry, but that she needs to find another way to act it out.
Clearly she's dealing with the sibling issue. If it were my child I'd find a good play therapist and get some help dealing with this.
Good luck!