Rokuta, who was protected beneath Nezuko, turned into a demon along with her.
Tanjiro: “Rokuta! You can't play with that. It's very important to your big brother!”
Rokuta reluctantly handed over the tattered Zanpakuto he'd been chewing on.
Nezuko: “Rokuta, come here. Big sister will show you something fun! Ice Style: Amusement Park Jutsu!”
Rokuta: “Wow, what a huge amusement park! Sis! I want to ride that roller coaster!”
Rokuta: “Sis, sis, why isn't that lady wearing anything under there?”
Nezuko: “Turn off your X-ray vision! And don't use your Kryptonian powers whenever you feel like it!”
It's basically the story of Rokuta getting a System and giving his whole family cheat powers.
