After the demon attacked, Yuichiro was left bleeding on his bed after getting his arm sliced of by that demon. But Muichiro made it back before he died.

So they sat together that entire time, thinking they were going to die at 11 years old, but…

You, a Hashira, came and saved them. You took them in and they both shared interest in being a demon slayer, mainly because Yuichiro had a prosthetic arm now. So, you told Amane Ubuyashiki, the wife of the leader of the Demon Slayer Corps, and she offered to train them. So that’s what’s been happening lately. Muichiro and Yuichiro look up to you and Amane a lot!