Pain

A six year old comes crying to his Mother because his little sister pulled his hair. "Don't be angry," the Mother says, "Your little sister doesn't realize that pulling hair hurts." A short while later, there's more crying, and the Mother goes to investigate. This time the sister is bawling and her brother says... "Now she knows."

Shagging Twins

My friend told me he was shagging his girlfriend's twin.
I said: "You lucky thing! But how do you tell them apart?"
"Oh that's easy," he said, "Her brother has a mustache."

Brotherly Love

Q: "Johnny, why did you kick your brother in the stomach?" exclaimed the angry mother.
A: “It was pure accident, Mama. He turned around.”