24(Demon Slayer)

An Au where Mitsuri and Obanai during a mission end up adopting(a survivor from a demon's attack) or with a child(due to a DBA) which they decided to try and raise together(besides them finally getting together...and this is probably before Giyuu meets the Kamados or the same day he meets them) and keep it a secret from the other hashiras until the child was used to their new life.

But while they were great parents, they had a hard time helping their child grieve their lost family as Mitsuri never lost a family member and Obanai hated his family so cue their surprise when after taking the child to play at a playground, they witness Giyuu help their child and comfort her after the child saw two siblings interact that reminded the child of themselves and their elder sister.

It surprised them since Giyuu normally kept to his corner and rarely interacted with others but he openly helped ObaMitsu's child besides seeming to understand them in a way ObaMitsu couldn't, of course that they were also surprised to see Giyuu in a playground as from what they and the other hashiras knew he didn't have children.

Cue another surprise for them when they see a pair of siblings(yes, it is the Kamados but Nezuko is still a demon but while burying the bodies and trying to clean the house, Giyuu and Tanjiro found the blue spider lilies and made a soup for them and Nezuko stole a bit from Tanjiro's plate and later that week while Giyuu was traveling with them they found out Nezuko was immune to the sun...Muzan has no idea about this...also the Kamado siblings and maybe the rest if the kamaboko squad will be younger then canon) run towards Giyuu with one of them they felt immediately was a demon but were in disbelief about it as she was in the sun.

At some point ObaMitsu approached the group and they and Giyuu stayed looking at each other for a few minutes before Mitsuri invited Giyuu and the Kamados to come to her estate so they could talk.

After eating and entertaining the children till they fell asleep, the trio opened up about the child origin and Kamado siblings(and a bit about Giyuu's past) which ended with Mitsuri offer as a babysitter for the Kamados if Giyuu needed and Giyuu saying that if they needed help with anything he would be glad to help.

In front of the other hashiras their interactions continued basically the same(except Obanai wasn't as antagonistic towards Giyuu but everyone assumed it was because it seemed like he and Mitsuri were finally together) but outside that ObaMitsu had grown close to Giyuu to the point the child called Giyuu their papa and the Kamados saw ObaMitsu as their parents too.

Of course that while Mitsuri would be more then happy to add Giyuu to their relationship, she knew Obanai still needed time to open up about his past to Giyuu and accept that he could and deserved to love and be loved by two people and that Giyuu needed to let himself move on from his losses and accept that while he may lose them, it was better to have loved then to live with the 'what ifs'.

At some point the trio get together and marry so they could officially adopt the children as their own(with Kagaya being who married them and helped with the adoption) but they never informed anyone else about it, not because they were embarrassed or possessive(well, Obanai is a tiny bit possessive) but simply because they forget and assumed that someone already noticed it at some point.

So imagine everyone's surprise when Tanjiro's trial(which happens later) comes and instead of just Giyuu defending Tanjiro and Nezuko, Obanai and Mitsuri also do it and are mad at Sanemi for hurting their youngest and at the others for letting it happen.

Anything else is up to the writer.