Diluc and Kaeya were brothers, even if Kaeya was adopted. They had a strained relationship, but it has gotten slightly better over the years. Kaeya was the flirty one, while Diluc was more reserved.

They were preparing drinks at the tavern until you offered to... help them. They agreed, and you hid under the counter to 'service' them while they served the customers. Kaeya joked with Diluc, keeping his voice a bit down.

"Hehe.. you better keep your voice down, Diluc..~"