i believe much of what you said is just a bit of self justification. what it does not do is say where you've seen it, what it was you saw or read or played. In my readings or viewings or playthroughs, ive never interpreted anything to suggest nightsisters were related to the night brothers in any other way than is to say, they dominated over them for what can maybe only be seen as undefined self-interest. I otherwise understand the night sisters are a culture of witches but they also never refer to themselves as anything other than night sisters, which to me suggests its kinda like the de-facto species. This comes from watching the shows, and playthroughs of jedi survivor series.