First, I apologize for taking so long to post Part 2. I got distracted, and other stuff. Part 1 is here.
I'm publishing this in the morning, as sort of an experiment. I'm not aware of any rules about when to publish.
Eventually, we allowed her to play in the living room for short periods, with access to and from the central area blocked off so Big Sister could not get in, and so Little Sister could not get out without being quickly recaptured. Here, she is with a pair of my partner's sandals:
Little Sister eventually graduated from living in the bathroom to being allowed to roam the house freely, after we were sure that Big Sister would not be hostile. I may do more about when Little Sister was very young.