25(Demon Slayer)

An Au where Kagaya instead of marrying Amane, he marries Tomioka Tsukato and they adopted Giyuu(since Tsukato is basically his mother as she has been raising him since their parents died).

How much would this change canon? Well, Giyuu may or may not become a demon slayer but if he does, I think he would still be trained by Urokodaki but take things slowly and be very careful in missions(so if he is to become an hashira, it would be after the others did).

Would the Ubuyashiki siblings still exist? Probably yes, they would simply all have black hair and have Tsukato as their mother.

Really, the chnage that would happen would be up to the writer.