The Chilling Adventures of Sabrina wasted Faustus Blackwood's twins, two supporting players who could have had clearer and more compelling roles on the show. Though Blackwood proclaims Judas as his heir and Judith "destined for greatness" earlier in the series, the Blackwood children are portrayed as his brainwashed followers and nothing more. Here's why Judas and Judith were so underdeveloped and underutilized.
Faustus Blackwood has diabolical intentions for his infant children at the end of The Chilling Adventures of Sabrina season 2, but these never come to pass. Upon poisoning his coven, he flees to Scotland with the twins. When Prudence and Ambrose track him down, they discover he's been in a 15-year time warp and Prudence's half-siblings are now childlike teenagers with no apparent abilities - even though Faustus can now wield magic other witches never thought possible. Judith and Judas exist merely as Faustus' sacrificial lambs and built-in disciples, despite an earlier intense buildup to their fates. Zelda Spellman even went so far as to hide Judith from Faustus.
The Chilling Adventures of Sabrina's showrunners have not confirmed why the siblings make such brief appearances, but it is likely due to the overabundance of other storylines. Perhaps with the show ending so abruptly, Judith and Judas' may have had better narrative arcs which ended up being cut for time. After all, it is primarily about Sabrina's struggles to navigate high school, Hell and the people in her orbit. Though Sabrina does help care for Judith when the girl is a baby, she has no contact with her or her brother when they've grown fifteen years in a month.
In fact, they are hardly touched upon once Ambrose and Prudence find them in Scotland and bring them home – Ambrose opts to put the twins under a living-doll spell, where they do not want for anything and live happily in a dollhouse under Ambrose and Prudence's supervision. Much later in Chilling Adventures Of Sabrina's third season, the story circles back to them briefly, showing Mambo Marie LeFleur, Vodouisant of the Fourth Ward teaching them the ways of their guardian spirits. This hints at the possibility of the twins uncovering their powers and eschewing their father, but this too meets a dead end.
Instead of having the children reunite with Faustus and worshipping his rotting corpse after Prudence beheads him, it would have been far more interesting to see them as powerful sorcerers, capable of vanquishing anyone who came near Blackwood. It would have been even better if Mambo Marie had helped unlock their gifts. In fifteen years of growing up with Faustus, Judas and Judith would have likely been exposed to the type of old magic he was learning. It wouldn't be surprising if he'd taught them to protect him at all costs. This would have also provided a nice conflict between Prudence and her half-siblings, who is privy to her father's emotional and psychological manipulation and knows Judas and Judith need release from his control.
Similar to a few other characters on The Chilling Adventures of Sabrina, Judas and Judith's storyline became too convoluted as soon as they aged up. It would have made for a compelling arc if they possessed as much power as Faustus did and teamed up against Prudence, thereby adding to the familial drama and upping the stakes. They could have been formidable opponents to the main group. Perhaps they will have bigger parts in the comic book continuation in lieu of season 5.