OK, this thread exploded with the new update, so I haven't read most of the last 90+ pages, but I'd like to put forward a theory which I haven't seen yet.
So; a popular theory is that Jill's sister is the MC's mother, mostly citing that they were both wealthy, died around the same time, yada, yada, yada. I still think that's bollocks. What I think is likely that they were separate people who knew each other and, more importantly, were close friends.
With the revelation that MC's mother ran away from home and stayed at a friend's for an unknown period of time, anyone else think it's possible that Jill's sister was that friend?