Kaeya and Diluc going firefly catching when they were younger. (They tried it once with Crystal flies but they were often hard to catch due to often going higher than fireflies do)
Diluc once broke an arm when climbing a tree to pick an apple that was too far up and Kaeya helped him back to the house while Diluc worked not to openly cry. (Said broken arm meant Diluc couldn't train for a good bit of time and it drove him nuts.)
Kaeya would sometimes sneak treats from the kitchen for him and Diluc to have.
Diluc and Kaeya often trained and spared together but also often had to take on different sparing partners due to the differences in their chosen weapons.
Kaeya tried a claymore once but found it too heavy for him to move like he likes too, the slower heavier hits more suited to Diluc. Kaeya likes being able to quickly move around his opponents, dancing around them and dodging their attacks. (He often used this to mess with Diluc since he's the faster of the two.)
As kids they'd race each other but Kaeya would nearly always win, Diluc would then demand a spar.
When they weren't allowed to use their training sword the two of them would get sticks to sword fight with. (Wooden swords weren't always allowed due to the damage they could cause)
When getting to have a food they liked the two would argue over who had the larger portion.
Kaeya and Diluc didn't start refering to each other as brothers till several months into Kaeya being taken in by the family. But the first time it happened both felt very warm inside.
Crepus would work to spend time with each of his boys but he was often busy due to the winery. As such the boys would often come with him to the winery, when they were younger the staff would have to make sure they didn't get into the grape vines. As they got older they would work to try and learn what they could of the business from their father, since one day it would be theirs to look over. Kaeya, however often picked up more and was better at focusing on the various jobs Crepus was in charge of, since Diluc's core goal was being a Knight and protecting Mond and his focus was more suited to matters related to his work as a Knight.
Once they were in their teens they'd work on Knight work together, helping keep each other on track and staying up late handling paper work together. If one started to doze off the other may do something like slap them across the back of their head, tease them, get them something to drink, or on more work heavy nights help the other to a bed or couch to properly rest.