Donald Faison played the exact same character when he appeared in Magic Joel (season 1) and Rumour Mill (Season 2); that's why he wanted to become a magician's assistant when he appeared in the mortal realm in Magic Joel.
Likewise, Lindsay Sloane was the same character in Sabrina and in Bring It On. She was so sick of being a victim that she dyed her hair and changed personality entirely when she moved. That or she accidentally drank a confidence/ambition potion.
The Evil Twin episode. The Judge says that "a good twin cares more about helping others than scoring points for herself". Katrina's use of magic shown in the montage suddenly reeks of trying to seem like a saint, hoping to convince people she is the good twin.
Multiple versions of Sabrina's friends (and enemies) disappeared from the show without explanation. This applies to the comic and animated series (Cousin Ambrose) as well as the live-action show. Considering the supernatural aspects of the show, it is tempting to think magic had something to do with it...
Every member of the Spellman Family has an Twin. And the Bad Twin is sent to the Twin Cities in the Other Realm. Cousin Marigold gives a clue to the family secret, so she obviously is part of the family secret, therefore her daughter Amanda does too. Of course we also remember that Amanda is for most of the series absolutely horrifying and spoiled. However Amanda is still around at the end of the series, so... exactly what was her Bad Twin like that she was the good one?
Well Amanda reappears in Season 6 much maturer. And as early as Season 4, she shows signs of growing out of her brat stage. Amanda does show Hidden Depths and by Season 7, is much nicer. It's not that much of a stretch that Amanda had matured by the time she worked out the family secret.
All children start out life as Chaotic Evil, and it's only as they get older that their alignment shifts. (Or, less flippantly, Amanda in her early appearances was a Spoiled Brat, and eventually grew out of it, as opposed to her evil twin, who would continue that behavior.)
Less positively, the discovery of the evil twin remains a secret until the witch receives their license at seventeen. It's still possible that she might be the evil twin.