The hundred-fold increase in child abuse associated with step-parenting has been a major theme of the research work done by Professors Martin Daly and the late Margo Wilson. They presented their work in their 1998 book, The Truth about Cinderella, and defended its genetic / evolutionary arguments at length in a review article [Daly and Wilson, 2008].
With more divorces and separations, spouses are being replaced by stepparents and "lovers," putting children at greater risk.
Daly M. & Wilson M. (2008) Is the “Cinderella effect” controversial? A case study of evolution-minded research and critiques thereof. Pp. 381-398 in C.B. Crawford & D. Krebs, eds., Foundations of Evolutionary Psychology. Mahwah NJ: Erlbaum