Although the show is a spin-off of Beavis and Butt-Head, the fact remains that it takes place in a different setting is a plausible excuse of their absense along with other characters from that show. In addition, Daria never mentions them or her previous life living in Highland, right after arriving in Lawndale, is for the better to articulate herself and her new surroundings since she obviously wants to deviate and forget about once living in Highland.
Why do the fashion Club accept Daria as Quinn's cousin,etc, even after her revelation as her sister in episode 1?
In episode one of season 1, Esteemsters, Daria announces to the auditorium that Quinn is her sister during her speech after being awarded her graduation certificate for her self esteem class. Throughout the 5 seasons and 2 TV movies, Quinn refers to Daria as her sister, the cabana girl, the cabana girl's cousin, the maid, librarian, charity collector, and other variations on these themes. However, Quinn denies that Daria is her sister for 5 seasons. The fashion club (Tiffany, Sandi and Stacey) all accept these titles given to Daria and do not question Quinn despite the revelation in season 1.In epsiode 6, season 5, "Lucky Strike", Quinn announces that Daria is her sister. Sandi appears to be shocked, as if she did not know. However, an explanation is finally offered as to why the fashion club appeared to have forgotten the events of the first episode. Stacey informs Sandi that she and Tiffany knew that Daria was Quinn's sister but were just being polite.Because of this, any goof/mistake listings are nullified aince the explanation is provided once all 5 seasons have been viewed.It also emphasises Sandi's naivety, which is also displayed in episode 9 of season 5, "Life in the past Lane" in which Sandi is duped by Stacey's part in the Ruttheimer magic act,