The full quotation goes like this:

They were of a respectable family in the north of England; a circumstance more deeply impressed on their memories than that their brother's fortune and their own had been acquired by trade.

In other words, the Bingley sisters remembered the fact that their family was respectable better than they remembered where the family fortune came from. I have no trouble believing that the Bingleys were a family of merchants, and not so rich, either! I picture Bingley's grandfather as the one who really made it big, perhaps as a brewer or a mill owner (just my speculation). I remember reading once that in England it was accepted that it took 3 generations to make the descendents of a self-made man "genteel", so if that was the case, Bingley and his sisters would be the right generation to do so. Although we don't think so, there was definitely upward (and downward) social mobility in JA's time - money did indeed (eventually) buy prestige.