"The fairy tale helps resolve this conflict by attaching and the bad (hated) aspects to another. The good mother oftens appears as a fairy godmother, the bad mother as a wicked witch or evil stepmother and the final victory of the former over the latter was the childs way to exorcise his feelings with the assurance that the godd mother is the real mother."
So, what this explain is the mysterious disappearance of real mother in fairy tale history. Every child upset with their mother thinks, "Obviously, my real mother would treat me better, so I must be adopted and this witch must be my evil stepmother!" Hence, the premature death of the beautiful kind mother and replacement by a wicked witch (and hopefully a fairy god-mother). I'm reminded of Prince Humperdink in "The Princess Bride" referring to his stepmother as "E.S." even though she was the sweetest thing on two feet. But, in every story Humperdink had ever heard the stepmother was evil.
The perspective's of Caldwell and Humperdink might explain the behavior of many stepmothers in Greek mythology. They have no choice, they have to be evil. Which explains Ino's over the top efforts to arrange the death of Helle and Phrixus. (I need to do more research here. )
* Note; this doesn't apply foster mothers who appear to be more like the fairy godmother.