I tend to think this exchange has more to do with Edmund's lack of concern for social niceties than a lack of concern for Fanny. It may also tell us something about Fanny's somewhat ambiguous position in the family. Sir Thomas had said early on that a distinction had to be preserved between Fanny and his daughters, but I get the impression that the family sort of works it out as they go along rather than the distinction being very clear to everyone. But I don't think Edmund considers it particularly important whether a girl is officially out or not.