That's a very good point! And one I most heartedly agree with. I have witnessed this kind of thing happening, too, with a brother of mine as well!

Obviously its nothing as serious, but him being the "baby of the family", or, the youngest immediate sibling, my parents developed a youngest child complex, where in they treated him as the youngest child, when he'd grown out of that ditch a long time ago.

Essentially, he learned that if he annoyed me to the point of retaliation, I'd get blamed because "He doesn't know better", or on the other side of the coin, "I'm older and should know better", and he got off Scott free - and still persists to aggravate me until this day with the same "you should know better" attitude, although my parents say they aren't treating us differently.