A particularly dramatic demonstration of this came on Saturday evening at teeth cleaning time.
As ever when "time for bed" is called (and the strops have ended), there is a mad dash upstairs to see who can be first to the bathroom sink.
In normal race conditions, it tends to be Jamie. And so it was again on Saturday. But this time his little sister had a plan.
Perched atop of his little plastic stage, Jamie committed the cardinal error of being smug. And Charlotte wasn't having any of that nonsense. Hell no bro. Action was required. And action was duly taken.
A split second later, with one swift yet clinical yank, Jamie's pyjama bottoms were instantly downgraded to a pair of ankle warmers.
Cue two contrasting displays of emotion; Charlotte in barefaced triumph, Jamie in bare-arsed meltdown.
Looking ahead, I now fear for the boy. In fact, I fear for every boy - which is kind of reassuring from a father's point of view.