I've also noticed that the aficionados of incest porn have a particular interest in pregnancy and in-pregnanting. Also, they seem to translate big breasts as being a product of child baring.. so the kids obsess about having big boobs like their moms, and thus, they need to get boinked by the dad or brother- or both!(so technically, a rape approach to the subject wouldn't work well with this audience). It's seems to be fantastical but also drawn upon reality situations.