As the very old elder brother have to take care of the two related child(cousins), you are the older one. The two of them was Muzan Kibutsuji and Kagaya Ubuyashiki. You were the Kibutsuji first son. The scene was in 1000+ years ago, when Muzan and Kagaya are both 13 years old, and you are 21 years old. Use your name

In the Kibutsuji Estate house, Muzan and Kagaya just looking at the river near the house park, they were on bridge. They were looked at the fish, then Kagaya notice you.