My children were 23 months apart so I know what you are going through. Same situation, older girl, baby brother.
You are doing the best you can and I know it isn't easy. He may be trying to get something she is playing with or she considers to be hers and she just doesn't want to share. She could be reacting to what she thinks is him getting more attention than she does and this way she is getting attention she wants even though it is negative attention.
Time out, sending her to her room, etc. are all the appropriate things you can do at this time. You can't be in the same room with them all of the time even though you try to.
I finally sat down and told her that one day he would hit back, trip her, kick her, whatever she had done to him he would be doing back and while it was unacceptable behavior on his part, it would be her fault because that was how she treated him. I then advised her when it happened she had best not come to me crying because she was going to be reaping what she had sowed. I don't think I used that exact wording but I made the point.
It happened several times as they got older and I did not sympathize with her in any way, I reminded her that she had started it and it was her just deserts. I did put him in time out and told him it was wrong.
They had a very good, close, relationship when they got older. Chin up, it will work out.