9(Demon Slayer)

An Au where Mitsuri's father was the one that found Giyuu when he ran away and so he and his wife decided to adopt him making it that Mitsuri and Giyuu grew up as siblings(almost like twins).

When Mitsuri's fiancé broke her heart, Giyuu almost beat him up but decided that starting rumours about him was the best to make sure he suffered while he comforted his sister.

It follows canon in Mitsuri deciding to become a demon slayer but while Tomioka wishes demons to die especially the one that killed Tsukato, he choose to follow Mitsuri but to be her support and always there from when she arrived from training or missions(so Sabito did survive and became the Water Hashira and the other hashiras haven't met him yet, not even Rengoku).

One day as the two siblings walked by a village they watched a  man throw a boy out of his house and shout about him being disgrace before closing the door, the two immediately run to see if the boy was well and after getting his story Mitsuri immediately felt compassion for him since Zenitsu, the boy, was manipulated by his fiancé like she was so she offered to take him under her wing wish Tomioka was completely in agreement with it.

Zenitsu soon was integrated into the Kanroji family(he met Mitsuri's parents and siblings) and was slowly being trained by Mitsuri when one day as he visited Mitsuri in the Butterfly Mansion, he saw a hurt boy training(Muiichiro) and saw a few looking worried about him so he decided to try and approach the boy which was hard but soon he succeeded and found out Muiichiro had lost his memories and could only feel that something was his fault for not being strong so he offered to take him to his siblings(Mitsuri and Tomioka) for help.

Tomioka immediately feels connected to Muiichiro and takes him under his wing even if it is Mitsuri who trains him as Tomioka while knowing the basic to defend himself from demons in case it is necessary(Mitsuri asked Oyakata for permission to get a nichirin blade for Giyuu) and just like with Zenitsu, Muiichiro found himself in the loving care of the Kanroji family and slowly healing(and maybe gaining a few memories back).

Would the hashiras the others find out about this? Yes, I think that during(or at the end) of Tanjiro's trial(Sabito would take Giyuu's place), Giyuu would appear and start giving a lecture to Mitsuri and Muiichiro for not telling him that Zenitsu had gotten seriously hurt from a mission and for them also not informing him that they would not be able to come home for a few days.

This would lead to a variety of reactions which I leave up to the writer as I leave the possible ships that could happen in this au.

Basically, I love when they compare Mitsuri and Zenitsu, Tomioka and Giyuu and also GiyuuMitsu friendship so I want them to become like siblings while also giving them a happier time in their life.