Wonderful though the ties of affection can be between sisters, I think that your answer of boredom is probably right. They can't be perpetually only talking to each other. JA tells us that the Bingley sisters like Jane and Elizabeth:

From chapter 6:
Miss Bennet's pleasing manners grew on the good will of Mrs. Hurst and Miss Bingley; and though the mother was found to be intolerable, and the younger sisters not worth speaking to, a wish of being better acquainted with them was expressed towards the two eldest.

I agree that Bingley does give indications that he likes Jane, but we don't know him before the novel. He may have called numerous other girls as beautiful as an angel before. In chapter 4 we are told that:
The manner in which they spoke of the Meryton assembly was sufficiently characteristic. Bingley had never met with pleasanter people or prettier girls in his life... If it is characteristic of Bingley to say that he had never met with pleasanter people or prettier girls, then he has probably said similar things many times before (implying a surprising monotonicity in improvement in the people he met - perhaps he spent his boyhood being raised by vicious gargoyles? :) ), so his sisters may have just discounted it as Bingley just being Bingley again.