Hi Eric3141 and

We don't diagnose people here. But I will compare what you describe about your loved one and what I understand about this disorder. Hopefully some of this will be helpful to you.
Last year dated a gal for 8 months. First 2.5 months best relationship ever. Treated me great. Then it all abruptly changed.
One of the formal diagnostic criteria for borderline personality disorder is: "a pattern of unstable and intense interpersonal relationships characterized by alternating between extremes of idealization and devaluation." One relationship does not a "pattern" make, but what this criteria describes is that if you are in a relationship with someone with BPD: they will love you (idealization) in an "extreme" way and then they will hate you (devaluation) with the same intensity. And whether they love you or hate you, will "alternate."
She said she had "abandonment issues" and not to do that again. Next time please tell her that I'm leaving. I knew her dad left them when she was 8 years old so I just chalked it up to that.
Another diagnostic criteria for a BPD diagnosis is: "frantic efforts to avoid real or imagined abandonment." And I think a lot of people see the efforts to avoid real abandonment: our BPD loved ones cannot stand being left even if they understand that it is because we have other commitments or responsibilities. In their mind, being left is the same as being abandoned.
The "imagined abandonment" is what I think confuses most non disordered people. As I understand it, people with BPD (pwBPD) will imagine abandonment even if we try our best to communicate the opposite. My observation is that for pwBPD, the closer they feel towards us, the more often they "imagine" that we will abandon them. So in a lot of these relationships, the harder we try to demonstrate our commitment and love towards them, the more often and more intensely will they imagine that we mean to abandon them.
Rather than just ask if we could sit there (which would be fine), she took her index finger pointed it at the chair of her preference then raised her voice and angrily said to me (as if I were a 3 year old that disobeyed a parent) "No, we will sit here!" It was crazy over the top angry.
Another diagnostic criteria is "inappropriate intense anger or difficulty controlling anger."
Another way I understand this behavior is as a reaction to their "imagined" abandonment. As I see it, pwBPD might end up dwellling on how they "imagine" our intent to abandon them. And even though we have no intention of leaving them, nor do we demonstrate any such intention; they still imagine us doing so behind their back. And wouldn't you be anger at a loved one if you felt they were being duplicitous?
She pleaded with me to get back together and I did as I cared for her very much and hoped for the best.
This kind of pleading not to break up is a clear example of "avoiding abandonment." You see it's only "abandonment" if we break up with them, not if they break up with us.
One because of extreme jealousy over the pretty girl serving us coffee at a shop and the other one seeming to want to intimidate me into doing what she wanted.
The jealous behavior could be a reaction to the "imagined" abandonment.
After the get back together, she asked if I could text her in the morning when I got up and at night when I went to bed so we could say "good morning" and "good night" to each other. Sounded good to me. But she had just made it my job to do the texting and reaching out... .something that persisted all the time. If I didn't reach out then I didn't hear from her. She stopped initiating. It became a one way street with me being the one to reach out. Not wanting to lose the great stuff got initially, I unwittingly kept giving and reaching out more in an attempt to get that back. Which it never did.
When pwBPD ask for such demonstrations of our commitment towards them, such as daily proof and consistent communication, it is not an effort to strengthen the relationship bound. If it were such, then they might reciprocate the behavior. No, I see such requirements as purely in order to placate their disordered feelings (of abandonment) which as I suggested only gets worse as they attachment towards us increases. So even if you do everything they ask of you, it is never enough to quell their fears.
She was selling her house so moved in with her sister. She then informed me by text that she would not talk or text all weekend (every weekend) as the house "had no privacy" but we could talk on her drive home from work which was 30 minutes. Very odd. Texting does not require privacy. Seemed to be both controlling and distancing.
Even though you have a very different kind of relationship with her as compared to her and her sister, I would argue that with your BPD loved one, she is getting to same thing from either one of you. So when she is "with" her sister, she has "abandoned" you. And when she is "with" you, she has "abandoned" her sister.
This is perhaps one reason why she wants such a clear boundary and distance between you and her sister. She doesn't want either one of you to be able to compare notes.
And I imagine, while she is with you, she might be complaining about her sister. She is probably complaining to her sister about you (devaluing you) when she is asking that you give her "privacy."
Is this BPD behavior?
You'll have to use your own judgement.
Best wishes,
Schwing