To be honest, I am still learning myself. I feel just like you, I'm confused. For the most part with she hits us or her sibling she gets timeouts. We know that most of what she does is impulse and she really can't help it, so we also try to disrupt that buy clapping, or we start to sing, or we do something we know will get her attention, and stop that behavior. We try many ways, you just have to find what works best with him. Just keep trying
Let me know if you have any more questions, I've been there and I know how you feel.