]Could LR really judge then how Anne's fatehr and sister would continue to treat her? Couldn't LR expect that Elizabeth would marry and leave as well?

This hadn't even occurred to me, but of course it is a very important consideration. Thank you so much for bringing it up!

Of course LR must have been expecting that Elizabeth would marry. She even expected that Anne would have married someone else by "now." It was never LR's intention that Anne would remain so long in her father's house (and with Elizabeth) and being treated in such a manner by both of them.