Looking at your responses, your sis in law seems more 'open or free' in her child rearing.
There's nothing wrong with that, but she's taking a path less traveled and that requires much more attention than what's considered the norm. Your niece can be easily steered into a pattern that doesn't help her fully develop in multiple areas.
People are choosing to take a different approach to raising their kids. For instance, we don't spank our kid - at all, we let him cry it out, and learn from his mistakes (as long as there's no blood we're cool). Our family thinks we're making a huge mistake, but we believe in other forms of behavior correction.