Nope, your niece isn't a special case. Kids often rise (or sink) to whatever expectations their parents set.
I have friends whose two kids ate the same food their parents did...they would occasionally try to get something different, because what kid wouldn't. But they never really expected "kiddie food". They sometimes had chips and such, but if mom or dad said healthy snack only, these kids automatically headed for the fruit bowl on the counter or the veggie drawer of the fridge, no whining. Their parents also removed them from restaurants and grocery stores if they misbehaved. They grocery shopped together, with one of them as the designated child wrangler. If one kid acted up, whoever was "on duty" would give them one chance to get it together then and there...if that didn't happen, the kid was carted off to the car for the remainder of the shopping trip.
No surprise that as these kids are hitting their late teens, they're articulate and responsible. Not perfect, but pretty decent.