Thank you especially to @GodamnCars @threeLocsts, and @KTheGrey; much more balanced - and yes, understanding - replies. Such a relief to hear, and respond, even though i was almost done and was away from laptop with a draining phone battery - and was very much repeating myslf, and yet still not being heard. I cannot help but wonder if this would have happened if my sister HAD had the nerve to post the question....
To repeat for those who are still making incorrect assumptions, even though I thought I was clear; the mental health decline and harm to herself was AFTER the taking of her daughter, it was NOT witnessed by her, and was exacerbated by the efforts she made - without support - to keep her child or even to stay in her life, albeit peripherally. She kept how she felt to herself, but crumbled after the two years it all took to be "finalised". And even then it wasn't - she had really bad legal advice. (I have seen the papers, so know the dates and how long it was allowed to take.)
The threat to use the CS for someone else's mortgage had the desired effect and she became the "bad guy" by paying the daughter direct, mainly for her to keep the pony on. The demonising of what she did. in some of the replies (genuine misunderstandings forgiven) is amazing to me. By the time she fell into reduced circumstances again, the pony was outgrown and sold on at a profit to another child and daughter kept the proceeds.
As far as I can see it, with my biased and regretful eye, he DID do her wrong, and preyed on her weaknesses (some of which I share....) and there had certainly been some emotional, physical and financial abuse (yet she still hoped for the marriage to continue). He took a beautiful, intelligent and career-oriented woman and turned her into a shell of herself. The issues that she had played a part, but he was not a person of any depth, so they would have confused and frustrated him - so SHE became "too complicated".
The grandmother was not in great health, but was tough enough to shut my sister out even when her own son had left his daughter behind and my sister tried to get her back (again, not through legal channels). She was by then occupying my sister's previous home, which made it all messier and long-drawn out than it needed to be.
In part by her own decision (shame, blaming herself etc ), and in part because both she and I have really bad "parents" and other "family", she had little to no support throughout it, with many not knowing, or caring even if they did know some of it.
One other comment with a bit of our backstory - although we have the same parents, I was not brought up with my own mother (my sister was; another contributory complication) and had very little connection until late teenage years, and beyond (and yes, it has failed miserably in the end) but I have never ONCE said that she "abandoned" me. Even though iit seems to be black and whte to some (most) on here, I do not consider it abandonment in it's most literal and cruel sense.
As for "raking over things" and it being "history", I do understand why some contributors want to see it that way. BUT, in terms of this (and some other aspects of our lives), I cannot help but be interested and want to know more. The lack of contact over the years makes it seem more recent, a bit like a wormhole - and some life events DO have a ripple effect for decades; why would something as big as this NOT have such a long-lasting effect?
And I do think that it is fair and right to give the matter time and careful considerration in an attempt to understand, even after "such a long time".
I don't expect everyone to feel or see it the same way - and I asked for input, and do not mind that so few people can see it the way I do. But again, I am so glad my sister doesn't know about the way this converstion went. I am saddened by it, and that there is such an ugly side to an otherwise very useful facility.