A.O.
123 Magic is an amazing tool to help control a little one. My son was very aggressive and wouldn't respond to anything including spanking, but he listens to this program. And the nice thing is, it takes stress off of you as long as you can keep up with it. Basically it is this...
"Son, please do this/don't do this". If he doesn't, all you do is raise one finger and say, "1, please do/don't do this." It is the only warning he gets. If he still doesn't just raise another finger and say "2", but don't repeat the order; you have already told him twice. Then do "3" and respond with an action. They have videos out with this program. They're boring, but they work.
It sounds like he may be looking for extra attention from you since sister was born. And any attention is not bad attention. Or a doctor may say that he's ODD. Either way, good luck.