Kaebedo is the unintentional best kept secret from Diluc.
I imagine that Adelinde KNOWs about Kaeya's relationship w Albedo but doesn't tell Diluc because she thinks he already knows, after all, he like collects intelligence for a living. And because the topic of Kaeya is seemingly off limits as well. And then when they finally start getting closer and start to make up, Adelinde of course thinks Diluc knows.
Kaeya hasn't talked about it with Diluc directly, but Diluc is always throwing jabs about how suspicous Albedo seems, so Kaeya also assumes that Diluc is just being an overly protective older brother and worried about the guy he's dating. Diluc means all of the things he's saying quite literally, so when he says something like 'quite an interesting friend you have there' he's not being passive agressive about the guy he's dating he's just trying to warm Kaeya about a suspicous individual.
So when Kaebedo kiss in front of him for the first time...
Diluc is VERY confused. When Diluc asks Kaeya about it, he's just like: 'We have a whole ass child together, Diluc. Of course we're dating.'
