"Now, how would you handle a boy that is slapping his younger sister's? He needed a little insight, and I gave it to him."
I don't wan't to be stuck in an argue over what the best solution is, only to hear I don't know what I'm talking about or that I don't know what's best for your kids. I do not know what's best for yor kids. But I do know that violence isn't going to solve world's problem. You can get upset or mad or whatever you want, but I don't think you have to lower yourself that much and slap him in the face. Didn't you felt sorry for doing that afterwards?
"My son is an adult and he does not seem to be scared by these two experiences."
That's good. But other kids will get hurt, because ther parents spank them. There is a reason spanking is forbidden where I live. And it is not bible-based.