What did Bingley's sisters think of the Bennets?

Mr. Bingley's sisters are snobbish. They look down on the Bennets as being from a lower social class. The Bennets are quite respectable, but they don't occupy the same exalted rung on the social ladder as the Bingleys. In this society, class is everything, and although Mr. Bennet may well be a gentleman, he's certainly not in the same league as Mr. Bingley and Mr. Darcy. Mr. Bingley's sisters are positively scathing about what they see as the Bennets' ill-breeding (they have a relative who lives in the unfashionable Cheapside, of all places!).
That said, Caroline Bingley takes to Jane Bennet, seeing her as somehow different from the rest of her family. Jane's pleasing manners meet with Caroline's approval; she certainly provides a welcome contrast to Mrs. Bennet and her irredeemable vulgarity. Even so, the prospect of Mr. Bingley getting engaged to Jane is still too much for his sisters to bear. Miss Bingley and Mrs. Hurst may like Jane, but that doesn't mean they want to be related to her by marriage. Simply put, they think their brother can do a whole lot better. In marrying Jane, Mr. Bingley would be marrying beneath himself.