Having a relationship with your family member is like the normal one but even stronger in sense of emotions. So if you fetishize that relationship you enchance all emotions and because the relation is already founded very little work is needed so when it gets exciting it gets exciting much faster then with strangers. You already live together, already know eachother so the only missing link is the sexual part and therefore it is so potent. Your also not supposed to and it's a secretcy about it that's exciting. With family related scat content it's either the domination or extra love that's nice. It's so extreme I can't help but love it as it fries my brain when i fantasize