My whole life I have suffered with mental health issues and I have seen how they affect others at times. I don't mean them too. If your niece does have NPD she may not know how or why she is behaving the way she does, not much comfort but a little understanding (I hope).
I have had 3 major relationships with women in my life. My mother and 2 wives all of whom are narcissistic in nature and I've only just found out what that means. I am currently starting recovery from another mental breakdown having discovered my 2nd wife cheating.
A comment from my sister about my second wife's 'gas lighting' caused me to look the term up and then move to narcissism - the initial description described my wife so well it could have been about her.
To my good fortune my brother in law's first wife is a narcissist also and he has been through the experience I am currently having. With the help of my sister and my father my Brother in Law has managed to shut down his ex-wife's controlling behaviour.
The basics of this is to stop playing their game. Only tell them what they NEED to know, think about each interaction before responding - quite often there is no need to respond and the narcissist does not like this, you can not change your niece but you can change how you interact.
I'm using these basic techniques and they are working, it's hard and it will be a long 'game' but results are already showing and my Ex is slowly giving me all the evidence I need to get the outcome I seek.
Also try and get some help with your feelings on your niece with some counselling or talk to some friends, but Listen (actually hear) to what is being said - it does help. I was closed for 47 years (even to myself) but now I've opened up things are changing - I am hopeful.
Good Luck with your situation and believe you can do it - because you can.
Loves and Hugs