] the younger girls who formed hopes of coming out a year or so sooner than they might otherwise of done. I'm not sure how old the young Lucas girls are, but it struck me that maybe Lady Lucas was more conventional than Mrs Bennet about the "coming out" of daughters the younger not coming out until the eldest was married?

We do know that Maria is already out, since she attended the Meryton Assembly and danced with Mr Bingley (see Mrs Bennet's list of Bingley's partners at the end of Chapter 3). So, there must have been at least 2 more Lucas daughters, younger than Maria (whose age we aren't given). Lady Lucas may be somewhat more conventional than Mrs Bennet regarding her daughters being "out," but perhaps even she realized it was not a good idea to wait for Charlotte to get married. :-)