You're welcome, tt!
I wonder... re your sister and her husband... if they believe in the idea that when one marries, one completely "gives up" one's personal identity for the relationship. I guess that doesn't explain what I mean very well. My mom passed along the set of ideas she had about marriage to me (obviously rejected piece by piece as I gained experience)... which were totally wack and simply don't have any basis in reality; what the real day to day demands, obligations - and JOYS - are. Something like how you are describing your sister's "fused" relationship...
so that in my marriage - I have no right to personal likes/dislikes, decision-making, boundaries of identity, no rights to ask for what I want, no life of my own outside of servitude, etc. That was her idea of being a good "wife". (and my exhusbands, too) Happiness and love and fun didn't figure into it anyway or anyhow... trust and loyalty and support was always one-way... and misery was all anyone could expect; it was one's lot in life for the security(?) of the marital relationship.
Or perhaps, it's the biblical "two become one" idea that they are trying to live out. Soul Twins, and all that. Regardless - it can all look and function like BPD; or like what river describes as the "master-slave" relationship, emotionally and psychically. It is mind-boggling to try to understand... there has to be a lot "suspension in the belief in reality" like some sci-fi movies use to get the viewer involved in accepting the plot-premises... and it can be just as scary as the worst horror story, too. It is, to my way of thinking - crazy, sick and insane. And it's diametrically opposed (and a perversion of) what we know as trust, respect, and the giving of love... in a real, mutually beneficial relationship where two individuals join in a committed partnership, yet remain themselves.