I'm a little confused, but let's see if I have it right.
Your son hit is little sister. Hitting is wrong. That needs to be reinforced. Can you listen to what he has to say? Absolutely. You can and should hear what he has to say. He needs to have his feelings validated, but then he needs to know that hitting is still wrong. Being upset about what little sister does is ok. Hitting her because of what she did is not. Then talk to him about what he can do instead.
Another kid (not related to him?) hit him. What should you do? Well, if the other kid is not your kid and/or not under your care (ie, you are not babysitting), then there's nothing you can or should do. The other kid is not your responsibility. Logically, you know that kids hit eachother. It's part of being a kid. That doesn't make it ok, and it's certainly something we want to teach them not to do. But it's still going to happen. No reason to be upset. Talk to your son, let him know that "Binky" should not have hit him. Possibly talk to him about the entire situation (why did "Binky" hit you) and what he might have done to contribute to the situation.
As I was typing my response, my oldest decided it would be fun to try and sit on his brother's head. Little brother didn't think that was fun and pushed him off. Big brother is currently in time-out, and little brother has been reminded that he can use his words and tell his brother to "get off" next time. Although, I can't really say that little brother did anything wrong.