On your argument that it is in a different setting, I quote Fontaine on chapter 28 "intra-family marriages were common about three or four generations ago", meaning it has been pushed out of normalcy.
Also, this is a fictional work created in the 21st century where it is common knowledge that *incest is not only illegal but also immoral to engage in because it biologically endangers children born out of such union. So it's normal that as we read this, we contextualize it with the real world.
Lastly, no it's not a different thing entirely and you just proved that by saying "it's a fictional story" and people would almost always couple it with "mind your own business" which you insinuated. Yeah, no shit but it's still illegal and immoral and I can call your action disturbing for it
I think it's important to note: I don't go out of my way to comment in a fictional work that is explicitly tagged *incest* and bash people there for having that kind of fetish. I know some don't grow out of their oedipus and electra complex/other complex and that's why even in porn, the "step bro/sis/dad/mom" is a successful genre (at least, the porn industry is sensible enough to keep it at "step" lol).
BUT PLEASE, this isn't that.
It varies. Some fujoshis may only see it as a "fantasy" but don't really support it (and I'm not judging, I know there are religious constraints that make it hard for them to support gay union), but others, I know, genuinely support it. I think that's when it becomes sad because it's a moral conflict that'll bother them especially if they are religious. I feel for them.
y'all disgusting for shipping her with her brother, go back to your hometown Alabama ヽ(`Д´)ノ