Kenrick, et al point out that "Familial provision of resources and care will follow the order (a) self > siblings; (b) own offspring > stepchildren"
It seems to me that the subconscious realization of (b) would be a logical explanation of the prevalence of wicked stepmothers and at-risk stepchildren in fairy tales (Cinderella, Snow White, Hansel and Gretel, etc.). It is found in the West, of course, but we also can find it in many other cultures, such as this classic of Indian literature: The Wicked Stepmother. Further, this Wikipedia entry, under "Fiction" lists many more fairy tales from around the world. "Hamlet" would of course be an excellent example of the wicked stepfather. Surely some work along these lines has already been done?