You are on the dot about Kiyoka’s biological mother not raising him. If you read the light novels or manga, Kiyoka claims he sees Yuri as his mom, for she was the one who truly raised him. For that reason, he was absolutely furious when his past fiancés treated her poorly, leading him to end these engagements himself. So, Miyo unknowingly got the approval from the mom on day 1. Smart girl. Also, in episode 1 of Season 2, he literally straight up tells Fuyu how he has never seen her as his mother.