Is sibling fighting normal?

So I was talking to my friend the other day, and I casually mentioned how my older brother still hits me when we fight like he did when we were kids. (Granted, he doesn't all out punch me with all of his strength or anything. He usually slaps me, so I slap him back.) She immediately got extremely concerned, and said that that wasn't normal. My brother is 21, and I am 19. I don't find it abnormal at all. Siblings fight after all, right?