Part of the problem is that you are trying to reason all the time with them. There seems to be this idea that we must treat children like little adults, and through talk & logic/reasoning then they will understand right from wrong. And that they will care. The problem though is that for small children they're still learning the boundaries in the world, and frankly they don't understand enough to care (as would an adult). Adults have learned that there are consequences (often rather harsh ones) to actions of stupidity or aggression. Children haven't, and to constantly shelter them from that sets them up for the fall.
A small smack for attacking (and i do mean attacking) their sister is a rather good introduction to those boundaries. An explanation & telling them how disappointed you are in them only goes so far. Basically it rests on how much they value your opinion of them. And yet you have to also remember that for small children time is relative. They fully expect others to forget if they have done something wrong ten minutes ago, simply because they do themselves. Therefore, the odd time, something more than a harsh word is needed.
There are good ways to use corporal punishment and bad ways. If done immediately after a child does something wrong, then it creates a buffer against it happening again. However if it is left for 5 minutes or later that evening when Dad gets home, the lesson loses meaning. Then its "just" a beating with no real relationship to the action. The child will already have lost the real memory of the incident and can attribute the punishment to just about anything under the sun.
As I said earlier there are times when corporal punishment are useful. Its a tool. A very effective tool if used correctly. But anyone can take the most perfect tool or device and screw it up if they don't think before using it. Corporal punishment should never be done in anger and always with a firm reason made clear to everyone involved.
Let me draw an example. I worked in a private school in China, and was farmed (hired) out to government public schools at different stages for extra money. The private school was where parents with money sent their kids in the evening after public school to get extra tuition in English. Public schools are free and can be attended by anyone. The private school students were treated like kings both by parents and teachers alike. We sucked up to them, because that was how their parents wanted it, and it showed in the behavior of most of the students. Some were lovely, but most were little ****s. The public school students on the other hand weren't allowed any amount of leeway in behavior. I've seen a female teacher hit open palm across the face of two students looking at their phones in class. The school supported the teacher as did the parents. The students were perfect in their behavior for the most part, and extremely respectful after school.
I'm not saying either way is better. A mix of the two would be a better set up. Children should be encouraged to grow, but as adults we should be there to step in when they're doing something foolish, or dangerous. And sometimes a slap is what is needed.
Also, as I said earlier, you seem to think non-corporal punishment means a quck no and moving on. Kid is smacking other kids? Weredid he learn that i wonder? Remove kid from situation and isolate. Simple. And yes you can explain to them why and tell them what you expect from them. A slap is never "needed" - as I have pointed out, it is possible to raise kids without slapping. In Scandanavia yo u can be fined for it, and yet... they don't have confused kids! I winder why?