It sounds like it could be some frustration from jealousy. Have you tried asking her if she feels angry and why? Even at that age some kids can express their feelings well.

If she is upset at the time you spend with DS, she is probably wanting some attention, and she is getting it by being told off, so she is actually being rewarded for the behaviour. Perhaps instead of negative reinforcement, you could try praising her and rewarding her for good behaviour. Perhaps if she is gentle with DS, or goes a while without hitting him you could praise her and read her a book or something, so that she is getting attention for good behaviour rather than bad?

Just an idea but it might be helpful. I well remember the days of 2 under 2 or thereabouts and it sure isn't an easy time!! It does get easier though.