It perfectly fits in with the setting of this world. Remember in the very first chapter, we learned that in their village it was considered normal to "cheat" on your partner. The farmer's wife was banging a neighbor outside at the same time he was banging another man's wife inside the house.There is something so wholesome that the sister is always involved in his Sex-capades. It makes the Harem feel like an actual harem and the Sister is just as big a degenerate as him, it just feels so weirdly wholesome, I love it.