It's really normal for little kids to act out whenever there's a big life change or upheaval.

Rather than telling her not to do something or threatening her, I suggest telling her quite clearly that you will not allow her to do that (literally: I'm not going to let you do that) And then stop her. You may have to run interference, or you may need to keep whatever it is that she's after completely out of reach.

If she cries or yells, I'd just let it go - it's her expressing herself and that's ok. But physical violence we need to be clear about. Set an example and be clear with her that you won't allow it. Punishment after the fact probably won't be effective as she isn't able to do as you say and she may need clearer boundaries at the outset.

Above all she probably needs time and understanding from you. She's little and her world has just changed enormously. It's stressful having a new sibling - kids are left wondering where they sit in all this and do they still have a mummy (or did they jsut lose mummy to the little brother/sister). Spend time with her as you can and involve her in day to day stuff with her brother so she can see that you're all still family and there for her.