My girlfriend and I have a 6 year old, a two year old, and one year olds. The six-year-old was from a previous relationship and the other two are from when we were together. We have been together for 4 years and my girlfriend constantly makes excuses for the six-year-olds Behavior. I'm an only child so I have no idea how siblings are supposed to get along and get that threw in my face quite frequently. But I don't feel like a six-year-old boy who weighs 70 lb should be punching a 26-pound little girl who is 2 in the face. But I'm told that this is normal sibling Behavior.