Looks like someone's pressed, bitch look I know it's everywhere, one of the reason why Japanese normal people hated weebs and anime lovers, because these damn writers tried to normalize this shit over and over again and how dare you call me r4cist when majority of the writers does in fact normalize this shit?? Being a law abiding citizen is what they want you to think that they are because we all know that a normal Japanese people no matter their culture and belief, they very much care about their reputation and manners in the eyes of public and honestly that's what makes them a good people. Do you think I'm proud of my country?? Hell yes but I was never proud of my country's dark side, if you asked what's the biggest issue on my country?? INFLATION, SCAM AND LIES. DO YOU THINK THAT MY COUNTRY DON'T HAVE RAPIST, INCEST AND PSYCHOS?? FACKING YES THERE ARE PEOPLE LIKE THAT IN MY COUNTRY AND I'M NOT AMUSE BY THIS SHIT.
TOOK ME 60 TO REALIZE??? HELL NO, I HATED THIS DISGUSTING BRO AND SIS COMPLEX SINCE VAMPIRE KNIGHT AND SWORD ART ONLINE.
THEY WERE NEVER NECESSARY IN ANY OF THE PLOT, THIS IS CLEARLY EVERY JAPANESE AUTHOR'S FETISHES
Well!! You can say it's rooted from their history/cultue , something really crazy must have happened there for them to turn out this way......... their normal is on a whole another level, it wasn't until 20th century that they felt, something is wrong, marriage among cousins, father marrying daughter if mother died, mother having sex with son if father works faraway was something I found after researching which was really normal for them infact if a married partner have sex outside of marriage, that's considered alright as long as partner comes back home [ (⊙_☉) ], there are many other things in their culture which personally I find weird but it is what it is Ig.
--I won't say this trope of incest doesn't exist anywhere in the world. e.g
West target wide range of audience so it needs to be politically correct and accepted by many, it doesn't mean there isn't stories like this, its just that they aren't adapted to main stream same for Korea and many other countries, they also have these just that not many(foreigners) know about it.
Anime/manga has a very niche media, "made by them, only for them" kind of thing but with anime popularity slowly people(foreigners) discovered this side too.
One of the theories was posted: there's high amount of one child family especially in otakus. People want what they can't get, so those people humor this idea, these people also don't have means to understand normal sibling bond and don't understand how people with siblings feel about incest, so to them having a cute imouto and developing a relationship might seem like an intriguing fantasy.
Second theory was: fetishization sells
Genere- NTR, incest, rape, Stockholm syndrome tropes etc are major examples
This is something you'll find in erotic websites, wattpad stories, LN, anime manga, it's more prevalent in Japan because making these is cheap as compared to making these in any other country so Japan can explore wide range of subgenres into mainstream, of different fetishes for their people who enjoy these.
One more thing I found is that---- their desires doesn't need to be symmetrical, you can desire something in 2-D but doesn't feel desire for that in 3-D world--- example those who feel moe for the sibling characters in manga, don't have siblings because if they had siblings then the reality of that would ruin their fantasy same can be said for maid character tropes, they can feel moe for maids in manga/anime but have nothing to do with women who are paid to do their housework, these people don't have maids otherwise it would ruin their fantasy, their desire is asymmetrical
There is explanation why this is even a thing in Morikawa's article and Patrick Galbraith, moe manifesto, one can understand an aspect of it, if interested.
Tbh I have read so many crazy stuff that now, my brain just ignore the things that I find uncomfortable to read for my peace honestly and I have come to deal with the fact that people have their own preferences which I don't care about as long as they don't force me to like something they like...
NOTE: not every Japanese like these but Japanese do like these. (this can also be said about people from other countries as well)

Honestly, wtf is wrong with Japanese people??? What makes them think that sister and brother complex is okay???? What makes them think that incest is okay???? Wake the fck up!! Like how many times do I have to watch an anime normalize that sh1t???