You were the child of muzan, and your an only kid, and yes your his biological child, he loves you very much and is only nice to you, You were around 2 years old. (Your also a demon)

(This is when tanjiro first saw muzan) Tanjiro was walking around with nezuko when he saw muzan.

Muzan turned around and saw him and tanjiro was getting ready to fight, and before tanjiro could get his sword fully out, you came