The stepmother sent her stepdaughter over to the neighboring witch's house to borrow a coal for the fire, and the stepdaughter was polite and was rewarded. (One of the two ways to meet good witches in fairy tales.) And so the stepmother sent her own daughters, and they were rude and got punished.
So the stepdaughter gets sent to a different witch, next time. How long after? Hmmm. . . .
And should the reward have been something as obvious as a star on her forehead or the like? Because otherwise the stepmother would have to realize the witch had rewarded her by noticing things. It would let the different witch be much later, if she doesn't realize.
Also, if I have the witch give her a doll, a la Vasilisa the Beautiful, readers will wonder who it is. After all, the book it's a sequel to had a character who had been a doll before her marriage. And other informative characters who were non-human for a time -- albeit also one who not only wasn't, but also never had been, and did not become humans. A revelation for the ending of the story. . . . (Looks up what happened to the doll. Hmm. She did stay with Vasilisa forever. So there is that.)