Camiella: Glad you are here and can so relate to what you're saying. While our situations are a bit different, much of the trauma and worry you describe I have lived with all my life. Know that you are not alone.
I have an identical twin who is a recovering alcoholic; has depression, anxiety, mild OCD, a problem with eating/food, and is diagnosed with borderline traits. She takes three or four psychotropic medications. She and I are middle-aged now and have been through a lot together since we were children.
My sister has never tried to self-harm, but she has been hospitalized multiple times for suicidal depression, had electroshock therapy which damaged her, and is on Social Security Disability. She goes through times that are pretty good, and then she has times that are scary and worry me out of my mind.
Like you, I have high anxiety about her. And I have deep survivor guilt that I have not suffered like she has. As an identical twin, I am never free of these. And I still have deep anger at our parents (including uBPD mom) for leaving us with a uBPD caregiver when they divorced when we were young, which without doubt created a "perfect storm" that hit my sister right where she was most vulnerable.
I am new here, too, and so relieved to have found people who understand, and such great insight and information. Maybe we can help each other.
