As you said, the sisters did not object to someone to talk to out in the country.

Miss Bennet's pleasing manners grew on the good will of Mrs. Hurst and Miss Bingley; .... By Jane, this attention was received with the greatest pleasure; but Elizabeth still saw superciliousness in their treatment of everybody, hardly excepting even her sister, and could not like them; though their kindness to Jane, such as it was, had a value as arising in all probability from the influence of their brother's admiration. (Ch. 6)

But more to the point, the sisters saw their brother's interest in Jane and wished to check her out. ("Keep your friends close and your 'enemies' closer.")

I'm sure, in addition, that Caroline's perception that Darcy was taken with Lizzy added an additional downside to having Jane too close, such that the sisters were not sorry to see the connection broken off when they left Netherfield. No meaningful friendship IMO ever existed.