No family dynamic is more ripe for drama than the sudden arrival of step- or half-siblings. Where older films would use this for slapstick rivalry (e.g., The Parent Trap ’s twin switcheroo), modern cinema leans into psychological realism.
In old cinema, step-siblings were enemies by default, with the conflict resolved through a shared embarrassment (the camping trip disaster). Modern cinema has replaced the "catfight" with the . my busty stepmother deprived me of virginity