theoatmeal.com is who did the drawing at the bottom.

i'll give you that. it WAS a bit muddy in the middle.

Usually it not that you're at a kids movie so much as an idiot brought a kid to a pg-13 or R movie.

While I sympathize with the brother, and even though his response and anger was very reasonable, he should NOT have hit his sister. Being "provoked" is not an excuse to act poorly. And when you use that excuse, you're trying to blame someone else for your bad behavior. Just because hitting or violence is an understandable reaction, does not make it ok.

Yeah, I disagree with the "just be perfect" logic here. Often, unreasonable/changing/unclear expectations come with parents who also use worsening degrees of punishment rather then figure out the cause or motivation of misbehavior. Just upping the ante on punishments isn't effective parenting. It's about control and anger. And to be honest, spanking is really a punishment that should stop around 10 or before if it is used correctly.
Parents have to figure out clear boundaries, expectations and punishments WITH the input of their children. An example would be if a 17 year old says out to late on Saturday nights. A discussion on why the parents have a curfew and why they feel the time is appropriate. Listen to the teenager and allow for rewards, like a later time after 1or2 months with no problems. Let children decide on what they think is fair punishment for certain misbehavior. Like loss of car privileges.