We all know Takeo died but…

What if he was turned into a full demon instead?

Everything goes mostly the same, Muzan attacks the family while Tanjiro is out selling coal, Nezuko gets turns into a half demon and Takeo getting turned into a full on demon. They have their interaction with Giyu and get sent to Urokodaki to train Tanjiro to be a demon slayer to find a solution for Nezuko and Takeo to be human again.

You’re Urokodaki niece and you live with him since your parents died, you basically take care of Takeo, Takeo seems calm around you too.