Sounds really hard. I have a 3 and a half year old girl and a 6 month old baby. What I have learned so far is
-even though my friend and I joke that it's all about "bribes" and "threats", it works and you can keep a sense of humor about it.
-use immediate negative consequences or "threats" of them. Use things he cares about and that are reasonable (if you trip your sister one more time I am canceling your play-date today or I'll have to put away your toy... Always, by the time I start walking to the phone to cancel the play-date due to her lack of cooperation, she consents)
-Use immediate positive rewards that he cares about. Ex: when he is nice to his sister you really celebrate and give him attention. "If you share one toy nicely with your sister today we can go for a walk to the beach."
-Be consistent
-Only choose "threats" and "bribes" that you are willing and able to follow through on, so think before you speak
-Lower your tone of voice, mean it, and follow through, without being scary, or course, or condemning him as a person. That is, say "We're not going to the playground until you clean up your mess" (natural consequence of his behavior).
-Get down to his level, give him a hug, look him in the eyes calmly, use humor, laugh at yourself, chase him around for fun to release tension, but follow through when you say that he can't have or do something.
GOOD LUCK!It can be exhausting. Make sure to take the kids out of the house a lot for social interactions for you, too, so your not isolated and can keep getting support from other moms..