This is my opinion only. But I believe the reasons that Evil 퀸 from Snow White and Lady Treamine from 신데렐라 mistreat the titular characters is different from the in-story explanation.
Evil 퀸 used the fact Snow was pretty as an excuse to get rid of an heir to the 왕좌, 왕위 which I presumable that the 퀸 was ruling for Snow.
Lady Treamine's daughters might be older 의해 a few months in the animated 디즈니 movie. And back in those days older daughters were suppose to marry before younger ones.
I don't really have a reason for Edward's step-mother, Narcissa. But I assume that the Kingdom is suppose to be run mostly 의해 a Queen?