I have always felt her presumption of inviting Maria and Elizabeth to stay a month longer at someone else's house but on this reread I noticed:

And if you will stay another month complete, it will be in my power to take one of you as far as London....there will be very good room for one of you--and indeed, if the weather should happen to be cool, I should not object to taking both of you, as you are neither of you large. (bold mine)

She offers to take one of them!?! What is the other to do? I have this crazy mental picture of Maria Lucas, with that deer-caught-in-the-headlights look the actress in P&P2 does so well, standing on the side of the road hitchhiking her way to London. ;-) But really, what did Lady C expect the other young lady to do, or did she just not care? And which one do you think she would take? She seems to take the most interest in Lizzy, so I assume the offer was particularly for her.