Re: OT from your post but...

... step...whatever incest isn't actually incest.

I disagree. First off, legally that is still defined as incest because the adult has the role of parent in the home. Secondly (and more importantly), I'm looking at it from the child's point-of-view, and to the child the adult caretaker(s) in his or her home is generally viewed as a relation, regardless of actual blood connection. Of course, it depends on how old the children are when they come into the home. If they are nearing pubescence then they will have a much harder time seeing the stepparent as a real parent, but if they're quite small they will almost certainly see the stepparent as a relative or quasi-relative.

Moreover, Pinker actually took pains to separate coercive incest from noncoercive incest in his book. As he goes on to say (from the paragraph I quoted): "Most of the rest are between fathers and daughters, and virtually all are coerced by the father. Some are between girls and other older male relatives, also mostly coerced."

Pinker does give sound reasons as to why some fathers might commit incest with a flesh and blood daughter, and what you find is that in a substantial number of those cases the daughter did not grow up in the home with the father; thus, the Westermarck Effect is much less effective or simply ineffective. In other cases the fathers simply ignore the Westermarck Effect. That might be truer in cases where the daughter is prepubescent, since in that case the daughter is unable to bear offspring anyway, and where the father has few scruples. Thus, the sexual reward trumps everything else.