YOU ARE WRONG. The retarded kid is the child of the dark-hair uke with his own mother and that's why the kid is retarded. I heard that when incest have sex and conceive a baby the baby end up miscarriage or retarded due to the similarity of DNA between both parents.
So u can say that the whole generation share 1 husband - the mother, the dark-hair uke who is the mother's son, the retarded kid who is the kid of the mother with her own son = all three of them f*ck one guy, the dark seme Kanou, meaning they all share 1 husband. just think like this, the grandmother, the mother, and the daughter all share 1 husband so for the retarded kid Kanou is his husband, dad, and grandfather at the same time. yes, it's a very disgusting story when u actually understand it.
**Spoiler Alert** I've just finished reading the 1st chapter..I think it's a bit confusing at first, but as the story's going on, I understand it or at least I think I do~ It's sad how the big brother want to get rid of that innocent lil' one just b/c Kazumi looks like their mother who has sexual frustration and had an affair with Kanou and even with the narrator, her own oldest son..I guess, though..and to top it all, these brothers love Kanou, and Kanou chose Kazumi. So that makes the narrator snap~~ Then, the story went on and ended like that.. TT^TT --So Sad-- Can someone tell me if I got it wrong ..?