Yeah even in korean scans the wording was a bit weird (english translations blew it out of proportion), but it'd still be hella weird if it was romantic because... why'd you put a literal flashback of him (an adult) and her as a baby like this just sounds like grooming, not just incest .. the implications are hella bad if it was romantic.
Ahan can get flustered like. out of any reason, really, and in the korean scans she still calls Mori her brother in the epilogue in chapter 566, something that has been suspiciously omitted in the english translations, hm (literally 2 chapters before the sus scene).
plus if they were really a thing since..well.. 17 years had passed, I don't think ahan would have used "older brother" honorifics anymore or straight up call him her brother in 566, but she still did.
For koreans due to the wording I guess it's up to "how well do you actually understand the scene" but I guess english speakers were set up for failure due to the weird ahh translations