I can understand this. My birth mother is now deceased and I no longer have contact with my biological siblings. My situation was different then your though, I was apprehended at birth and in and out of foster care, my adoption was finalized when I was 5.5 years old. I had memories of my birth mom (all the memories is about the abuse she inflicted on me) and siblings. My birth mom's death was very hard on me, it came almost exactly 6 months after my mom's death. That was the start of my latest bout of deep depression.

In your situation with having your birth family be supportive of you I would probably tell. My parents are deceased but my siblings know of my diagnosis, not sure if they know what BPD is.