Hi angry aunt,
You are not alone in arriving here really angry. I came here the same way. I arrived furious at what my SO's (significant other's) uBPDxw (undiagnosed BPD ex-wife). She made false allegations of child abuse, had a parental alienation campaign going on, was using her children as spies, neglecting her kids, lying etc... .I'm sure you know the routine.
I've found the members and information on this site truly helpful.
I'm going to tell you what someone told me when I arrived that I found helpful... .shifted my thinking.
Focus on your niece and
not on her mother... .I know easier said then done but give it a try.
You won't be able to change her mom she is going to do what she is going to do. What you can do for your neice is be a different example of what an adult woman can be and you can listen to your niece and validate her feelings.
You might suggest Therapy for your niece if she isn't receiving it already. My SO's daughter's both had dysfunctional behaviors they picked up from their mom too. We call them "fleas" around here. Therapy was helpful, they felt heard, learned coping skills etc.
You might want to check out the "Co-Parenting" Board for more information on how to support your niece, you can also vent to a group that has been there

Panda39