you're his older sibling.

you were dealing with the other two kids, zenitsu and inosuke, when you heard the hashira scream.

you look over and see the hashira physically unable to do anything because of how hurt he is

you look at your brother and see that the hashira's screaming because gyutaro has his hand over the hashira's wife's mouth and is looking at her with a very vicious look. you also see another demon slayer about to try and behead gyutaro without him noticing