It IS quite confusing! I had also always thought Bingley to be the middle child, but now I'm wondering if I was always thrown off by the wording! ...I found two instances where Mrs. Hurst is referred to as Bingley's "eldest" sister (once in Ch 3 where the party first arrives on the scene, and once in Ch 8 during a card game). My question: if Bingley had only one sister who was older than he, would she have been referred to as his "elder" sister and not "eldest"? This seems the logical rule, but then again, you never can tell with JA's own unique writing style! :)