Yes, Kaeya and Diluc are actually brothers in Genshin Impact, but not blood-related. According to the lore, Kaeya was adopted by Diluc Ragnvindr's father, Crepus Ragnvindr.

Ultimately, Kaeya encountered Diluc's rage. But as the two fought, a great cryo elemental power surged through Kaeya, stunning both of them. That's how Kaeya was bestowed with Cryo vision.

Genshin Impact character Kaeya invites Travelers to share a drink with a ... Kaeya is known for being an excellent liar and has a rather tumultuous relationship with his Pyro-aligned brother Diluc.