So sorry, this is a long one but I needed to talk about it bc your shota art is doing things to my brain.

I hc Kaeya being about two years younger than Diluc.

That being said: Kaeya getting brought into the winery, 7 year old Diluc was immediately lovestruck. He would always seek out physical contact, Kaeya just always thought this kid that was now his Gege was simply very affectionate...Kaeya didn't know any better so he got used to it.
Dilucs favourite times were when Kaeya came to his room to sleep in his bed because had a bad dream or it was raining again. He could unapologetically cuddle and smell Kaeya then, under the guise of consoling and calming his little brother.
One of those nights, Kaeya just turned 6, it was raining heavily and thundering. Kaeya who had snuck into Dilucs bed earlier was shivering and whinsing at every loud pang. He got so scared he had an accident...Diluc immediately jumped into action, telling Kaeya to take the dirtied clothes off and change into some of Dilucs. While Kaeyas back was turned, Diluc eyed the wet panties in his hands with curiousity. After a quick look over his shoulder he secretly sniffed the pee soaked panties. It changed something in Diluc…when they lied back in bed that night, he had a warm fuzzy feeling in his tummy and between his legs it felt throbbing…
With 10 Kaeya did notice some of his used discarded panties missing and then turning up again two week later, but he neither really thought anything of it nor did he actively question it. He also didn’t question when with 12 now 14 year old Diluc still would relish in shared bath time and even offer to help Kaeya clean. His Geges touches would always feel almost too gentle and focused at times but Kaeya would always push it onto the “Diluc is just very affectionate”-Stance.

Diluc on the other hand now, was rolling in conflicted feelings. Shame, for using his brothers used panties to jerk off, having to shift his legs to hide his growing dick when Kaeya would call him Gege and touching Kaeya with ulterior motives during bath times, while Kaeya suspected nothing. Lust, for wanting to defile his little brother and hear his pretty voice shout in lust and arousal through the whole house. Responsibility, for Kaeya was his little brother he had to keep save…but can he keep Kaeya save from himself?