So Shiny Days keeps having Sekai and Setsuna say that "they are family because their moms are related somehow" and that their mothers say it's complicated.
Well, time to go back to the Overflow family tree.
Ok, so Sekai and Setsuna have the same father but in the case of Setsuna, her father is also her grandfather.
Also, Sekai's mother is the sister of Setsuna's grandmother.
Yeah... I'd also just tell the kids that "it's complicated".
Makoto having only 2 canon kids make him pretty damn tame compared to the other protagonists.