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."

Like Mother Like Daughter

A man takes his 10 year old daughter to the doctor. He says "Doctor, I want to put her on the pill." The Doctor says "Why?!? Is she sexually active?" The guy says "Nah, she just lies there like her mother."

Oldest

Dad: "You ask too many questions. Just like your older brother used to."
Me: "But... I'm the oldest."
Dad: "You are now"