Plus their relatives so that's wierd
Can I just clarify here that Kotaro and Ayumi ARE NOT ACTUALLY RELATED. They use "relatives" as a go-to so Kotaro doesn't have to stress about people finding out they're in a relationship.
Kotaro looked up to Ayumi in the past as a brother because lil' Ko needed someone to rely on while his three little brothers relied on him. It was an admiration for a cool older man that developed into love.
But rest assured, Ko and Ayumi are not blood-related in any way, shape, or form.
http://www.mangago.zone/read-manga/fushidara_na_hanatsumi_otoko/mi/099e3966120641c6472c6cb93c8b6403_chapter-1/8/
He says relative of my mother, even if it is distant, they are related, unless Kotaro is not actually related to his own mother.
Also, for curiosity's sake, why don't you like AyumiXKotaro?
Personally, I'm glad Kotaro got to be with the man he loves. I might've been okay with Kyujo having a thing for Ko, but Kyujo ruined it when he said that he simply wanted Ko so he could "make him his woman." That is just blatant disrespect for Ko, something that Ayumi never treats him with. Ayumi treats Ko as both an equal and as his precious uke, and they're happy together. If Kyujo finds someone new (Chief Sakurai, as various readers are hoping for, for example) and treats him well, then that's all well and good. But Kyujo and Kotaro is an absolute NO for me, that's for sure.
They are relatives
Kotaro x kyujo personally I don't like kotaro with ayumi