FWIW, I didn't remove the incest tag.
The
Westermarck effect should still be at play here and these two would not actually develop a romantic relationship. The psychological phenomenon has been shown to apply beyond immediate family members to include children you grew up with in close proximity (e.g. orphanages or step-siblings).
I have no objections to restoring the tag. It's not a blood relation but it is pseudo-incest.
I've had to do it on several incest and inseki manga now, but the Westermarck effect is actually something that was debunked. There is no biological avoidance of incest, but rather a cultural mores one; if you grow up in a culture that hates incest, you are more likely to hate incest and actively reject it, originally taking social cues from adults and then society at large. That doesn't mean that the cultural mores are absolute, merely that's where the effect that was observed actually comes from.
Also, while I'd be in support of an inseki tag, they are definitively not brother and sister by blood, only raised together, which means that there's only a minor cultural scandal at play that they were socialized as such before suddenly revealing that they weren't actually related, rather than anything greater like the taboo which the incest tag is for.
People don't understand how good the payoff is for a tsundere character when done right.
They don't understand it because of how bloody rare it is. Most depictions are hyperviolent rage machines that flip and do violence against their prospective partner because someone ELSE looked at them, or similar.