I'm so sorry that your daughter is unwell with BPD and that your family is in such turmoil. I understand the situation well because I have lived it.
My daughter fell ill with OCD and anxiety at age 13 and the impact was devastating for everyone. She was seriously ill and at first none of us really knew how respond. There was such a mix of emotions, frustration, helplessness, sadness and at times anger.
My eldest son was scared (if his sister could fall then anyone could, including him) and missing my time and affection. He started acting out to get my attention. My husband was in denial and we fought constantly. And everything that was happening was a secret because of the stigma surrounding mental illness. I just wanted to disappear.
The best thing I did was to find a counsellor, actually a mental health nurse in private practise. She taught my husband and I about OCD and coached us on best to respond to issues/situations in real time. It's really important that you and your husband become a united front.
We also had sessions with our son, which meant we could help to reduce his fear and ensure he too felt loved and supported. Importantly, the three of us became an effective support team for my daughter.
It's really tough but it can get better. I know that it did for us. People do unfortunately fall ill, but they also get better. And with professional support you and your family can make a big difference to your daughter's recovery. You are absolutely doing the right thing to reach out for help.
I also want to encourage you to look after yourself in all this. Take some time every day to do something you enjoy--something to nurture your soul.
Hang in there.
Kind thoughts to you