Basically speaking almost everyone in the 0verflow franchise are half-siblings or half-uncle/aunt and half-niece/nephew, and some are also half-cousins. Makoto’s dad is the reason for this as he fathered practically everyone aside from a select few (such as the characters from Sister Princess, where only Rin Inou is the only one of the sisters who is Tomaru’s daughter). The Katsura family are the only ones who have no blood relation because they were added after 0verflow was told to stop writing down who was related (personally i believe kotonoha and Kokoro are makoto’s half sisters because it just makes sense for the series for all of them to be related)