What is the mutation that occurs in our genes when a sibling is born? That has to be the only explanation for why perfectly angelic children turn into little demons if it will cause their own flesh and blood any amount of anxiety or pain. Doesn't matter if they are the younger or older child. Seems like their motto is "If it will make him/her cry, let's do it!"

I guess the bigger question is, why do they actually think it is logical to come to a parent to resolve when they have tormented each other to extreme.