"She used to threaten suicide to get what she wanted, and she was a cutter."
Sounds like whoever diagnosed her didn't realize that this goes with Borderline Personality Disorder.
She sounds like an HPD/BPD case to me and if thats true she needs the DBT therapy that the BPD's go through because the HPD therapy by itself will not fully help her.
Anyway as for you I believe like myself and many other victims of these people you are probably suffering from Post Traumatic Stress Disorder which needs its own treatment to overcome most times so you might want to look into that for yourself.
In the meanwhile though while you think about that you could do what I've done and sign up for the excellent excerpts of the book "Tears and Healing" if you don't want to buy it outright.
The author of it Richard Skeritt details his healing from his destructive relationship with a BPD partner when he was unaware of what she was during the relationship and helps others to do the same.
Yours is a sibling relationship true but I think the book may still help you regardless.
http://tearsandhealing.com/
You may also want to pick up Emotional Vampires and Without Conscience as these may help you as well somehow.