Consider the 1700s/1800s, and the number of small, isolated communitites. Consider the difficulties of travel in a cash-poor society.
Availabilty of eligible partners. For all that cousin/cousin marriage isn't incest, a few generations of that and you can easily wind up with genetic problems.
Hey, homo sap is a mammal, right? The problems are more obvious with such mammals as show-dogs.
Actual incest, I think, is more a cultural thing, far more likely in physically or socially isolated communities. (Based on casual readings, not on scientific knowledge on my part.)
Art