Just wanted to let you know a story with a happier ending:
Knowing what I know now, my ex step-mother was BPD. Pretty much no doubt about it (I just didn't understand her behaviours back then).
I didn't grow up with her but my younger sister did, from when she was 4 until 8 or so.
I just wanted to let you know that she's now a well adjusted 19 year old, happy, healthy, emotionally mature and sensible. Great sense of humour and no obvious damage done from those 4 years with the step-monster and her 3 daughters (the middle one had many issues, more than likely a BPD in the making although the other 2 seemed ok).
So yeah... . well done to you for breaking that cycle

x