...IMO this is a perfectly socially-acceptable way of saying that he and Elizabeth are "better friends" because they are now family, and just one of the "many pretty things" he says to everyone. Don't forget that Elizabeth had already made him feel uncomfortable about Darcy *before* the militia left Meryton, and I'm sure he knows full well that they won't be seeing each other for a long time after he and Lydia leave for Newcastle!