Hello Mazi,
I want to join DreamFlyer99 and GeekyGirl, in a warm

I am so sorry your family situation is so stressful... .Trying to support a loved one w/BPD can be very confusing and frustrating. When we get to know the disorder, and understand what drives the behaviors, it gets easier to not take things personally and to focus on using the tools that help us be more effective in communicating with our pwBPD (person w/BPD). As a result, our lives get more peaceful and we are often able to help our loved one as well.
It does take time to learn about the disorder and the tools and techniques, but it is well worth it in the long run.
I understand your struggle... .Our situation is similar. I've been married for over 11 years and my step-daughter is 32. She's been married also and has 3 children. Luckily, I have entered the picture long after the divorce, but have born the brunt of her anger and jealousy nonetheless. She was always very close to her dad, and even though she is married, she kept an emotional 'claim' on her dad as well. As she needed our help over the years, she learned to 'tolerate' me, but I have felt that there was never a connection between us. We certainly did not like each other, even though I love her and am comitted to supporting her because she is my husband's daughter. In the last two years, we experienced some bizarre behaviors and found out about BPD. At the time, she was also blaming her dad for every trouble in her life and for the divorce etc. He was devastated by her behaviors and also did not know how to deal with her... .
As we learned more about BPD, we changed our approach (it IS counter-intuitive at times); and after a while, she slowly calmed down a bit and reached out to her dad again in a positive way. Fast forward eight months, we have had them visit at our place, we have visited them, and are in contact - things are stabilizing, and my relationship with her is actually better than it has ever been... .
Not every child will respond so well; there are no quarantees, and we may also in the future experience some set-backs. But no-matter their response, your own lives will be more peaceful nonetheless.
You have found a good place with wonderful resources and members that understand what you are going through... .
How has your step-daughter responded to her diagnosis? Does she know that you know?