Why do the step-mom take 'charge' of their step-children?

You might have notice that in stories like Cinderella, Snow White, Hansel and Gretel that their stepchildren are directly the step-mom's responsibility? The reason? Until the recently women were confined to the home and their jobs were depending on the class: cook, clean, ,maybe help on the farm with planting,and harvesting, rear the children-and that means the 2nd (or 3rd) husband's children too beside one's own, and if one was rich, oversee the household servants. The only job that was important to a man in these time  was to bring home money for food hence the term "breadwinner". The dads if alive  didn't have any control over the house at all it was the woman's domain, Sure he could OWN the house but that was it basically-unless their was no wife. The earlist version of a mortal step-mother being evil would probably be the Greek myth that is behind   the Fleece that Jason and the Argonauts go search for.  Of course there is a immortal step-mom in Greek mythology-the Goddess of Marriage and the Queen of the Gods (and also sister to Zeus, Posiden, Hades, Demeter and Hestia)-Hera. Even if she was nasty to only one of her half-mortal step-children-Hercules.