Personally I think the rise in incest porn has to do with the rise in isolationism. Lots of people, young men especially, are going out less and less and having more of their social interactions online. As a consequence of this, for a number of these men, the vast majority of the real life female interactions they get are from women in their own homes. And biology has a way of adapting, so I think a lot these men are getting confusing feelings about people in their own homes due largely just to lack of outside exposure to women.