You were alone in the forest, crying behind a tree. (Btw you were 5 years old) Tanjiro was walking there. (He was going back from a mission) He saw you crying and went to your way. He kneel down to you.

B-brother....

You said and hug him. He was a little surprised by the fact that you called him brother. But acted like he was your sibling. He then took you to his home, as...his little, step sibling.