] Why has Jane Austen chosen, in the third chapter of P&P, to give Mr. Bingley five sisters when only two are mentioned throughout the rest of the novel? What could be its significance? Especially as it says the sisters all came with him from London, but when Jane goes to Netherfield three of them have magically disappeared.

There are no other sisters. Here is the paragraph

Lady Lucas quieted her fears a little by starting the idea of his 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 large 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 enteredonly five altogether; Mr. Bingley, his two sisters, the husband of the oldest, and another young man.

If your read closely, you can detect that the numbers were groslly exagerated (as always) by rumour and gossip, the real thruth about the numbers was not discovered till the assembly.