When he does something that ends up hurting his sister, how do you address it?
Specifically, do you focus on the thing that he just did? Or do you focus on how his sister feels?
The latter is very important in helping a child learn how to appropriately set boundaries, and how to read the emotions of those around him. It's far more important that he learn to read when those around him are uncomfortable, sad, or upset, rather than to learn not to do specific actions - even when they are "never do" actions, unless they're actually dangerous - because once he learns to read the emotions of those around him, and accepts that it's important, he'll stop doing the things he shouldn't do.
Five year olds are also typically very self centered. Hopefully not as much as a two or three year old, but they're still well on that side of things. This means that one of the things he needs to be learning here is that other people's feelings are important.
So, when he (let's say) hits his sister, or gets a bit too into a physical game and hurts her, the best approach in my experience is to show him his sister, and point out her physical cues that show to him that she's feeling (whatever she's feeling).
Then help him see how his action led to that emotion, and help him find ways to avoid that action in the future. Avoid any sort of comments like "You're a bad boy" or otherwise suggesting that this is anything other than a single, specific action that he has control over and can do differently next time.
Junior, do you see your sister? Things got a bit out of hand, and now look at her. She's crying, because she's hurt, because you hit her on the arm there. Can you see how sad she is?
But mommy, we were just playing tag.
I know, Junior, but when you play tag you have to be careful to keep things in control, right? Tag is fun as long as you don't hit the person you tag too hard. But you got a bit out of control, right? When you get excited, is it easy to get out of control?
We were having fun, mommy!
Yes you were, and tag is a lot of fun until someone gets out of control, right? If she'd hit you that hard, you'd be pretty sad and not having fun, wouldn't you? And you probably wouldn't want to play tag for a while, either.
(pouty face with big lip)
Why don't you go give your sister a hug and apologize, and next time do you think you can focus on not hitting too hard when you tag?
Sis, I'm sorry I hit you. Can we play tag some more? I'll stay in control, I promise.
