You're not the first to be confused by this passage, Katy! This is actually JA's sense of humour at work - she perfectly describes the way gossip works in a small town:

Lady Lucas quieted [Mrs. Bennet's] fears a little by starting the idea of [Bingley's] being gone to London only to get a large party for the ball; and a report soon followed, that Mr. Bingley was to bring twelve ladies and seven gentlemen with him to the assembly. The girls grieved over such a number of ladies, but were comforted the day before the ball by hearing that instead of twelve he had brought only six with him from London -- his five sisters and a cousin. And when the party entered the assembly room it consisted of only five altogether -- Mr. Bingley, his two sisters, the husband of the eldest, and another young man.

Lady Lucas starts the rumour (quite likely based on fact) that Bingley has gone to London to fetch a group of friends to visit him /attend the Meryton Assembly. The local grapevine expands that into 12 ladies and 7 gentlemen, and then turns it into five sisters and a cousin (who don't exist). As it turns out, Bingley has only two sisters. The writer of P&P1 also took this joke seriously, and Mr. Bennet announces with a straight face that Bingley has five sisters.