I don't agree with not allowing him to be within her reach. It is important that she realises that he is part of the family. The more you try to restrict her, the more she will be determined to do things to him.

You need to think of this from her perspective. You used to be her mummy and she got all your attention. Suddenly this "thing" has appeared that is loud, takes up everyones time and attention AND gets to have his comfort dummy all the time where she only gets it when she sleeps.

I am not saying that she should be let loose, but it's important that she gets to spend time with you and him in a positive way. Singing songs, reading stories etc are good ways to do this.

Also try not to say she is a "big girl". she needs to know that both her and her little brother are equally loved by you.

Kids are smarter than we often think, she'll get there - it's all a transition. Good luck - it's a tough road sometimes!!!
Our midwife likened it to if your partner suddenly brought home another wife/girlfriend. How would you take it?