Hi Determined25,

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I'd like to welcome you to bpdfamily. I'm sorry to hear that. I can see how painful and disappointing that would feel when you're denied access to your niece when you mended a falling out. Many of our members can relate with you and offer you guidance and support, you're not alone.
HOW can I diffuse her and maintain a relationship with her so that I can be in my niece's lives? I just want to be able to put past arguments in the past and move on.
She's triangulating which is common with a pwBPD/NPD, BPD is a persecution complex and the person believes that their circumstances are caused externally by others, a pwBPD will cast themselves as victim and others a persecutor, the way to diffuse her is to identify triangulation and to not participate in this dance, remain in the middle of the triangle and don't take any sides, if you take a side it keeps conflict and blame ongoing. That being said, she's triggered with feelings that she can't cope with and projecting that on someone else, which is you at the moment, it's not personal to us, I suggest to depersonalize the behaviors. Here's an article on drama triangles.
Escaping Conflict and the Karpman Drama Triangle