"Mom! Her paper is touching my FOOT!"
"She just ran past me and whapped me in the head on purpose!"
"Stop LOOKING at me!"
"Well, fine! Then I don't want to play with you EVER AGAIN."
"Aaaaaargh!" (door slam)
I guess even the best sibling relationships have their moments of dysfunction? For example, I can clearly remember being furious with Uncle Awesome in the base comissary when I was about eleven. I can't remember what his offense was, although I can assure you that it was annoying in the extreme. What I can remember is my hand literally ITCHING with the need to punch him. I also remember weighing the swift and terrible punishment my mother was sure to rain down on me versus the satisfaction of wiping the smirk off his face. So I guess I get it.
What it's hard to communicate to your eleven-year-old is that someday, years from now, you're going to think your little sister is really cool. Even though today she seems to be Satan.
Or convincing the eight-year-old that the condescending despot posing as her older sister will someday be her best friend.