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.
In "Jenny's Non-Dream," it's revealed that every single one of the Other Realm's laws has to have a loophole—in fact, that's a law (Rule 803) in and of itself. Of course, some people might claim that since every law has a loophole, there must be some exception to 803. And they're right—the lack of a loophole for 803 is 803's loophole.
The Friday the 13th law that witches can tell mortals without consequences. There's an episode dealing with witches voting whether or not to tell their mortal spouses about their powers. It's likely the Friday the 13th thing is there to test out mortals' changing attitudes towards magic.
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.
Well remember the season 7 episode where she is horrified that she is the cause of Sabrina's future death. The twins are led to believe that pushing their sibling into the volcano will kill them. The test shows that a good twin doesn't have the heart to do away with their evil twin. If Amanda shows remorse for accidentally killing Sabrina then it seems like an indicator that she's good at heart. The evil twin wouldn't be sorry at all.
Hilda, Zelda and Sabrina are considered weird for their fondness and respect for mortals. Now factor in all the Mind Rape, manipulation, lies and total violation of privacy and personal control they've inflicted even on mortals they loved. How badly do normal, and especially predjudiced, witches treat mortals? Hell, Hilda has caused hurricanes and set off volcanos because she was mad at dates, which is mildly irritating to a witch but would kill countless mortals. Is the whole series secretly really dark?
The premise of the Season 2 episode To Tell A Mortal is Sabrina's secret being revealed to the mortal world and it is Played for Laughs. But has anyone wondered what would have happened to Sabrina if Valerie HAD chosen to betray her to Libby and the media?
They would all have forgotten immediately at midnight. The magic of Friday the 13th says that any mortal told about the secret will forget it as soon as the day is over. Of course Sabrina would still know that her best friend sold her out - but that's part of the drama of the episode.
The season 3 episode "Sabrina and the Beast", where Harvey Kinkle serves as Aesop Collateral Damage and is turned into a beast. The spell Cousin Susie uses changes him slowly over the course of a day or two. Think about it, most spells in the series work instantaneously. Kindly Cousin Susie decides the best way to teach Sabrina not to judge people by their appearances is to have Harvey go through an unexplained (so far as he knows) and prolonged transformation from teenager to dumpster-diving animal.