I am sure it probably has at least SOME to do with her being a teenager. Teens can be little @$$holes, pardon my French. They can grow to be angsty little b*tches who think being cruel is funny. It wouldn't surprise me if she is going through enough of her own crap and thereby it makes her feel better to tear others down.

That is still no excuse as far as I am concerned. Even as a teenager you are old enough to know better than to treat people so disrespectfully. But, sometimes teens are just that way. Hopefully it is just that. It would stink to think that her toxic parents have corrupted her as well.

I know it isn't exactly easy, but best not to even let people like that bother you. I mean in general, as I can surely understand how you once sweet niece becoming a little jerk can be pretty upsetting. But, in general people who seem only to want to spread their negativity are much better ignored. Don't even allow them the power to tear you down. Instead, focus on building yourself up, and focus on being above that BS drama.