You don't develop late onset BPD, so it is more likely your mother has always been BPD. BPD is not a black and white disorder... It exists on a spectrum, and the severity of it can vary from one person to the next, and it vary over time for a same person.
My BPD mother went from very severe symptoms when she was young: alcohol made it much worst. At some point, she stopped drinking, and was diagnosed with depression. She took antidepressant and this period was calmer. She would still rage, but it was nowhere near the emotional dysregulation she had when she was drinking and not taking any antidepressants. She did better for a long period, her boyfriend stayed with her a very long time and this stability seemed to provide some sort of comfort, decreasing the dysregulation. She was still a bit odd and my H would say :"your mom is intense.", But she wasn't rageful... More... Drama oriented.
Recently she took my grandmother in for a while, and my stepfather is very sick... And she got completely dysregulated when I went over there to visit and stayed there a while. Her symptoms were in full effect, minus the alcoholism.
All this to say : she went through periods of high energy and high dysregulation, and through periods of calm... What always stayed is the drama orientation, self-sabotage tendancies, constant shifting in self image, and some kind of emotional intensity very specific to her. There were outbursts, but also intense love and clingyness. Intense love bombings... Followed by complete devaluation.
Under stress, the dysregulation is often made worst, while stability helps reduce the severity of symptoms. And your mother seems to be under lots of stress right now.
Also, it is true that BPD mothers do not always treat all their children the same. You might have been shielded from your mother's borderline if she was mainly using your sister as her emotional punching bag. I was my mother's main punching bag. Both my brothers thinks I exaggerate how she is, because she never raged at them as much as she did me. They were boys, boys are worth something. I was the only daughter, and she replayed her own trauma on me.
As for the vulnerable narcissist tendancies... You will likely find that your mother correspond best to a specific diagnosis, such as BPD, but still, she will exhibit traits of narcissism, even maybe histrionic disorder. BPD is "Cluster B", and as such, it exists on a spectrum with the order disorders as well...
Don't worry for the length of your post! Writing is essential to your own processing and understanding, so write as much as you need to...
