I've read this novel about 58,002 times and that thought never occurred to me. Duh! That's me back to P&P 101! And Adrian's reading is reinforced a few lines later by the narrator's comment that "The rain continued the whole evening without intermission: Jane certainly could not come back". "Her sisters were uneasy for her", evidently because the rain came "soon" (i.e. sooner than expected), so that was a chance of Jane's in fact being caught in it.

Having thus acquitted the lovely Elizabeth of any insensitivity towards the health of her sister, however, I'm still left with her parents' equivalent attitude when they find out what has happened. I can't see how Mr Bennet's reference to the possibility of Jane's developing a serious fit of illness, or even of dying, can not be based on some kind of commonly held expectation that that possibility is a realistic one. The comment wouldn't work otherwise.