The popularity of the “stepsis/brother” porn lies on the concepts of “proximity” and “familiriaty” (in terms of frequency, as temporal regularity). This works because our mind is already wired up by our sexual instincts. So, when something “really close and frequent” happens, the possibility of sex (which is pleasant) triggers our excitement even disregarding whether we are into or not the “in***t fantasy”. Weird, huh’ (?)