I never thought much about the sisters until a recent re-read. They're not just obstacles; they're the entire reason Cinderella's situation feels so hopeless and claustrophobic. Their constant belittlement and the sheer pettiness of their demands—making her pick lentils out of the ashes—create that atmosphere of domestic imprisonment. Without their active cruelty, she's just a girl with a dead dad living in a big house. They make the magical intervention necessary. The ball scene is the obvious turning point, but for me, the more interesting influence is earlier: their insistence on looking perfect for the prince is what creates the chaotic, rushed environment where Cinderella can finally voice her wish to go, almost as an afterthought they don't even hear.
Their later recognition of her at the ball, followed by immediate, furious jealousy, directly triggers the second half of the plot. If they'd just been vaguely mean stepsisters who didn't go to the ball, the slipper test doesn't happen. Their specific blend of vanity and desperation is what makes them try to mutilate their own feet to fit the slipper, adding that layer of grotesque dark comedy. In the end, their influence is the push from a sad home life to a full-blown fairy tale requiring external salvation.