My BPD stems from childhood abandonment (absent father) and persistent emotional abuse from my likely NPD mother. I learned to doubt things I feel/thought and often reality sometimes. Real love and emotion connection were terrifying to me on a subconscious level. I never could trust anyone, not even myself (the only one to trust was my abuser). I developed so many coping mechanisms to protect myself from my mother that involve projection, distortion of reality and emotional manipulation. My brain learned all these horrible ways of protect myself from my abuser and I applied those behaviors to everyone around me.
It has taken years of unpacking my childhood trauma to get to like 90% remission of symptoms. Before that it was years of denial about my behaviors before I even started to entertain the fact I had BPD.
I'm sorry to hear BPD is effecting both you and your brother. There is hope but he has to be on board and it's a long road.