20(Demon Slayer)

An AU where due to a demon's blood art, a child that looks almost completely like Sanemi except with black hair and their eyes' shape appears suddenly during a Hashira meeting.

Of course that the reactions are first to make sure the child isn't a demon and that Oyakata is safe but soon the child explain what happened and presents themselves as Shinazugawa ???(the name is up to the writer as is the child's gender).

They all know that Sanemi only has one alive brother so the child in front of them can only be Sanemi's future child which is quite shocking as no one expected him to have kids in the future(plural because the child mentioned having siblings, adoptive older siblings and younger blood siblings) but soon they started to try and guess who could be the child's mother.

Of course that Sanemi wasn't present just like Kanae and Giyuu since they were on a mission(the one were Kanae would meet Douma but in this one the three of them kill Douma and no one died) so they couldn't use observing Sanemi near any female as a way to find out so thye had to go for all female slayers they knew he had interacted with.

Those from the lower ranks were taken out as Sanemi only interacted with them during missions or they were too young so that left all the female known hashiras(Kanae, Shinobu and Mitsuri...and yes the word 'known' was used for a specific reason).

Seeing the last three options it was no doubt that they all guessed that the child was Kanae's kid, even Shinobu believed that, as Kanae was the only one they knew that could calm down their angry hashira and make him enter reason.

Unfortunately the child had been more interested with telling more about the demon he was fighting to Oyakata and interaction with the Ubuyashiki children so the child was not asked who their other parent was so the hashiras kept with their assumption.

Moving time forward, the three missing hashiras finally returned from their mission with the good news of having killed an uppermoon and of course that at this point a few hashiras(meaning Mitsuri, Rengoku and Uzui) had asked the child their age and when they were born so they calculated that the child was made around this time.

So everyone after hearing the news of the trio defeating Uppermoon 2, they assumed that there was a celebration and Sanemi and Kanae slept together and while Shinobu wasn't happy that Sanemi slept with her sister before they even started courting she would leave it be since she came to find the child endearing(just like her other parent...but they are all dumb).

The trio first ended in the Butterfly Mansion were their injuries were treated(thankfully nothing life threatening) and then they went to tell Oyakata about how they ended up encountering Douma and killing him.

After that, Uzui being the God of Festivities didn't lose time in making a two in one celebration were the trio defeating a uppermoon would be celebrated but unknown to SaneKanae+the child it was also a celebration of their future relationship(and yes, no one told the trio about the child and miraculously the child and the trio haven't encountered each other).

The celebration happens, everyone outside of the children(the child and Muichiro) and Himejima got drunk and most do not remember what happened but keep their assumptions(that SaneKanae slept together).

Until the next hashira meeting that happens two weeks later and has the child present as they believe that soon they would return back to their time and they wished to see their parents before that happened.

Imagine the hashiras surprise when the moment the trio sees the child Kanae starts laughing like crazy while Sanemi and Giyuu blush so much that they resemble a tomato as this reaction made no sense to them.

And things got even more crazy/confusion when the child called Giyuu, who they believed to be a man, 'mama'.

It was after a small reunion of the child with their past parent and past Aunt Kanae that the child finally returned to their time leaving the trio(mostly SaneGiyuu) to eb questioned.

That day, the other hashiras found out how much oblivious and dumb they have been to not know that Giyuu was a woman, Kanae and Sanemi saw each other as siblings and that SaneGiyuu have liked each other since they met when they were children but got separated due to their canon stories(Tsukato dying and Giyuu running away+Sanemi's mother becoming a demon and killing most of his siblings, etc).

Anything else is up to the writer.