Why most stepmother are evil bitches?
The one in doctor Elise isn't evil. But Yeah, most of them are evil for the plot and character development.
My guess the whole evil stepmother probably originates from the fairy tale Cinderella and from their other people have reused that cliche but made the "evil" stepmother cliche more dramatic and extreme. (╯°Д °)╯╧╧
Granted I am sure Cinderella was not the first but being one of the earliest forms of writing that used I would not be surprised if that is where this trope was started and from their has been reused in hundreds of other forms of writing and genres.(⊙…⊙ )
Go gurl show that bitch of an ass stepmom who's the boss.