Same old story, big step-sister shows little step-sister a handcuff trick, little step-sister tries the same trick and is, well, tricked. Big step-sister is a sadistic -head so that's not the only pair of handcuffs she adds, and little step-sis is trussed like a hog and kicked around, because that is what big step-sister's do. This scene is kind of iek that study they did where random people got to be prisoners and guards, and they had to stop it early because humankind is, at the heart of things, a roiling mass of chaos and power-dynamic issues.