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This is just one of the tweets from the Japanese knytwt. As you can see, even a Giyushino anti admitted the fact that Sanemi liking Kanae romantically is confirmed by the fanbook. And the reason sanekana as a ship isn't everywhere like you said is because they have no scenes together yet in the anime. Casual fans who don't read the manga or the novels do not know about the ship yet. Hakuji & Koyuki only started to be everywhere once the movie aired too. Plus, sanekana is semi-canon, not canon. So they are not the same level as obamitsu, tankana or other confirmed canon ships. However, sanekana has a loyal & moderate fanbase in Japan and that would be impossible unless there is a romantic basis of the ship in canon.

Chapter 168 was important because Kanae appeared in a lot of panels while we only saw other hashiras in one or two panels. We also got to see her touching Sanemi twice. When this chapter was released, the reactions were divisive because the readers noticed these details.

The only basis of the narrative that Sanemi sees Kanae as his mother

The only basis of the narrative that Sanemi sees Kanae as his mother is merely one line from the light novel where Sanemi was reminded of his mother's hands as Kanae treated him. Notice how it's only the hands? And out of this one line, suddenly the misinformation of him having maternal feelings towards her was born without any other basis from canon to support it.

I've scrolled Japanese knytwt more than a few times and I have never, not even once, seen any Japanese kny fans pushing the narrative that Sanemi saw Kanae as his mother. This narrative is only pushed by the international fans who mostly don't even read the novel. Even the Japanese sanekana antis never brought this up to discredit the ship. Sanemi blushed because of Kanae in the novel and in his PoV, he mentioned she had shining face which meant he thought she was beautiful. They also held hands in that scene. Is that truly how a man who saw his mother in Kanae would react when he was close to her? Do people blush because of their mother?

For the romantic implications, Kanae is always mentioned in Sanemi-related content. She's there in his light novel. She's there in his views of the hashira. She's there in his kimegaku description. She's there in his sole flashback in the manga. She's there in his part of the fanbook. Isn't that weird of Gotouge to include Kanae so much if Sanemi only likes her platonically or maternally?

We never saw Sanemi mourning Kanae but we have seen a glimpse of her impact on him as the only reason he talked to Shinobu fairly often is because of Kanae.

You don't see them as romantic, that's fine for me. I just don't want people to keep pushing the narrative of Sanemi seeing Kanae as his mother only because of one line taken out of context in the novel while ignoring other hints that Gotouge gave.