When they all first arrive in London (Chapter 26) "Mrs. Palmer and two elderly ladies of Mrs. Jennings's intimate acquaintance, whom she had met and invited in the morning, dined with them."
We know from many other posts in other boards that it was polite to invite someone to dine with you the following day, because it's assumed you already have your dinner planned for the current day.
Is Mrs. Jennings being kind in inviting two ladies on the very day she meets up with them? Maybe she knows they will eat better at her home than her own? Maybe since they are elderly, she invites them on a day when they're up and about and well enough to come?
I don't think she is being thoughtless here, because, though sometimes a little vulgar and gossipy, she is kind and would surely be enough of a housekeeper to know the 'next day' rule.