Growling at humans is a behaviour that should be nailed with an immediate hard correction, so the dog is very clear in its head that it overstepped the mark & it will not be tolerated.
However if the dog generally has exhibited a steady, stable temperament, it would be useful to know what the dog, sister & older child were doing when the incidents took place.
Has the dog been taught that the human not the dog is in control & that its place in the pack is below the humans? Has it been obedience trained, given boundaries, taught bad behavior causes negative consequences & obedience is rewarded with praise? Is the dog healthy or could there be an underlying medical reason for the behavior?
More detailed information would assist in providing a more useful answer.
"Multi BIS (local) GreatGrowkenn Agnes CGC" is ranked #5 (all-breed) in Best Malaysian Bred Dog Of The Year 2010"