Hello Chai and welcome to bpdfamily, I'm glad you found us, you are not alone.
So sorry to hear what you dealing with your niece and have hit a wall with her recent nasty communication and as you describe a possible cry for help from her sister? To the right of the screen are tools and lessons that help us better understand and learn how we are able to change our behaviour and communication styles for positive outcomes. Take your time, as I and many here learn this is not a sprint, small steps work. Have a look the lesson 'know good v bad triangulation', just wondering if that may help you consider both of your nieces? Does your BPD niece fear abandonment, maybe that is what your non BPD niece is trying to express on her behalf?
How old are your nieces, are they living at home, near you, sounds as though they are emailing you? How has your niece taken the diagnosis, you say it was a while back, has she accepted or is in denial, is she in treatment?
You are doing the right thing, educating yourself - educating myself about my daughter's emotional disorder (yes I'm still learning over 12 months in) and working through the positive changes I can make for myself are the greatest gifts I can give my 28yr daughter, same for her loving family and friends.
I'm wondering how the parents are coping? You'll read here how important it is we care for ourselves first to be able to walk with our loved ones.
Take care, of you too, and keep posting - always helps

WDx