So let me get this straight:
Guy called Tomaru Sawagoe has several kids with his half sister, let's call their children generation 1.
Generation 2 is made when he has a daughter with his daughter, Kagura Inou.
He then fucks his grandaughter/daughter for gen 3.
This goes on and on until he has a son with his great-great-great grand daughter Rika Inou.
And in the midst of all this he goes around banging other people and somehow also manages to be related to all the main characters in School Days except for the Katsura family.
Guy is a legend, may also explain why School Days can have such screwed up endings.