I get why people search for evil stepsister stories – that trope taps into a specific blend of domestic horror and betrayal that’s hard to find elsewhere. The 'twist' element is key; it shouldn't just be about a nasty sibling from page one. The best ones lull you into a false sense of normalcy, maybe even make you sympathize with the stepsister, before revealing the rot beneath.
I'd argue the peak of this isn't in strict fantasy Cinderella retellings. There's a gothic suspense novel from a few years back, 'The Last House Guest' by Megan Miranda, that plays with this dynamic in a modern, non-supernatural way. The 'sister' figure is more of a chosen family, and the betrayal cuts so deep because of the intimacy that was built. It’s less about petty rivalry and more about a calculated, long-con kind of evil.
For a more classic, dark-fantasy take, 'Stepsister' by Jennifer Donnelly flips the script entirely. It's from the stepsister's perspective, questioning who gets labeled 'evil' and why. The twist isn't in her actions, but in the reader's understanding of them. It's a redemption arc, but one that starts from a place of genuine cruelty, making the journey matter.
If you want pure, unapologetic villainy from a stepsister, the 'A Court of Thorns and Roses' series has Nesta Archeron, who for a long stretch of the fandom was considered irredeemably cruel to her sister Feyre. While she's a biological sister, the dynamic hits all the same notes – the resentment, the emotional coldness, the feeling of being a burden in your own home. Her later development is a whole other conversation, but for the 'evil sister' experience, early Nesta is a masterclass.