Lupita,
Yes, it sounds like your sister-in-law shows traits of BPD. Just the way you described her "flip flopping" "can't figure her out" gave me the red flags to say that she must have those traits.
You mentioned that you want her to like you. I remember feeling that way about a lady who was BPD too...cause when we were together, she seemed to ADORE me...like I was the ONLY friend for her...yet, as soon as I did anything to help her over her issues, suddenly she would DISAPPEAR. She was molested as a child and learned how to keep the family secrets. When I dared to "blow the whistle" in some situations, I was quickly seen as "ALL BLACK" and she justified ending the friendship. This happened more than once with her, so please keep those boundaries FIRM, Lupita. We care about you!
You went through some really AWFUL things in your past. I am SO SORRY! You didn't deserve to be hit or put into a situation of helping someone to beat you...that was a very SICK, EVIL man who did those things...not a "father" by any stretch of the definition.
Now it's time to begin telling yourself who YOU are. You are a good person. loved by God, worthy of being loved and respected and honored.
Rootin for ya, Lupita.
~Laura
P.S. Ami NAILED it when she said about the D and N mother. So typical for the child to watch the mother's reaction or cues in order to decide a response. I'm currently counseling a lady who is in her 50's that still watches everyone she is with, in order to decide how to respond or what words to answer with.