Seriously Demented | BPD Mother Replacing Daughter

What do you think of this?

First some facts:

• My mother divorced my birth father in 1968 due to secretly reviving her relationship with her high-school sweetheart
• My mother's 2nd husband adopted me and all evidence of my birth father was wiped clean (baby album, photos, birth certificate, etc)
• My birth father remarried and had two kids (one boy and one girl)
• My mother divorced my adopted dad in 1980, marrying my adopted dad's best friend with whom she had an affair
• I don't have a relationship with my birth-father and his kids
  
• My mother and I have had estrangements through the decades, and we are presently in an estrangement

Recently I discovered that my mother has struck-up a relationship with my half-sister (my birth father's daughter), and my step-father (my mother's present husband) just took my half-sister on a tour through his old job site this past weekend. I find this very odd, disturbing, and confusing.

More odd, disturbing, and confusing is that my half-sister is calling my mother "Gramma" and my step-father "Grampa"... and referring to each other as relatives. What!? To reiterate, my mother divorced my birth father in 1968, which he then remarried and had two children (one of which is my half-sister who was born in 1974). My birth father was erased from my life when I was only months old in 1968. When my mother divorced her 2nd husband (my adopted Dad) in 1979, my mother tried to push a relationship with my birth father in order to hurt my adopted Dad. I didn't peruse a relationship with my birth father, but my mother has always managed to stay in touch with them (my birth father and my half sister) in order to tell them all sorts of bad things about me when my mother and I are estranged.

At this point, with my half-sister calling my mother 'Gramma' and fostering a relationship as 'relatives', the relationship has crossed yet another dysfunctional line beyond all in the past. Even going to the point of taking her kids to see my mother, driving across states, and spending the night at my mother's house is simply ludicrous.

Replacing one person for another (my mother replacing me with my half-sister) has a psychiatric term but I haven't been able to find it. If you know to what I am referring, please let me know.

This whole circle of dysfunction is mind boggling... DEMENTED.