In Episode 16, when Kagami, Tsukasa, and Miyuki decide to visit Konata at her part-time job, the latter asks them to pick up the latest release of a couple of manga. At the bookstore, they are served by a character who doesn't appear to be the "standard" extra seen in the series. Even after the three girls leave the cash register, the shot lingers on this character who is joined by her sister. All this seems to suggest that this is something of a cameo performance or a hat-tip to something else.

Who are these characters?