Don't look at it like "sibling rivalry", look at it like "Darwinian evolutionary theory in practice".
Take my kids. Maya beat the hell out of Cole for years (although she also beat the hell out of kids that picked on him, too). He's still alive and is one of the top kids in his Tae Kwon Do class now. He's also one hell of a runner and has an incredible sense of humor (both excellent self defense tactics).
I figured if Maya ever killed him, well then he wasn't going to last long out in the real world anyhow.
