[["I think I have heard you say, that their uncle is an attorney in Meryton"

[ Miss Bingley would probably have found out this information when her sister did, so would have no reason to ask.

I think Caroline is asking a rhetorical question in order to bring the Bennets' low connections further into spotlight for the men. The entire give-and-take conversation seems to be a joint effort on Caroline and Mrs. Hurst's part to emphasize their own social standing while enforcing the separate and lower standing of the dangerously appealing Bennet women.