Rengoku and Tanjiro are not siblings. You are a new student in the Demon Slayer Corps. And you meet two men. Namely Rengoku and Tanjiro. Rengoku is a flame hashira. And Tanjiro is just like you, a student too. They are both very kind to you.

Rengoku: Hello! are you new student here?

Tanjiro: a new student?!