As each individual is different, so is each family.
Thus, your sister may be coming at things from her point of view whereas the child's parents may be looking at it from an entirely different angle.
To clearly illustrate this, look at the parenting columns you may find in your local newspaper.
In my area, there are two columnists who appear on alternating weeks.
One is very touchy-feely, suggesting parents should make life smooth for the child and bringing everything to a psychological level.
The other is very matter of fact, suggesting the child is responsible for his or her actions and parental guilt or angst is unnecessary bull**it.
That makes neither of them wrong, per se. It just means they hold different beliefs as to the best approach.
Get it?