I've found this site really helpful in dealing with my pwBPD. I'm glad your therapist introduced us!
Causes me panic attacks thinking about what’s going on, but not being able to do anything about it
What exactly causes you panic?
Is your sister in treatment, or has she ever been?
Many of us can relate to the chaos that one family member can cause. We talk about boundary setting and other useful tools. Probably most helpful to me was actually reframing the way I understood BPD and what it does. Understanding it pragmatically meant I didn't have to take the manipulation personally. It increased my empathy for the person's struggle, and on the flip side, gave me clarity about what boundaries were needed and I enforced them without guilt.
Change is possible. Hope is possible. It takes work and sometimes it doesn't feel good or fair, but we're here and we get it.
pj