
Incest is bad because marrying into or breeding with someone genetically similar to you can result in birth defects.
If you aren’t blood related, even if it’s weird, it’s not fundamentally wrong.
Atleast that’s my stance on it. As long as you aren’t hurting anyone in the process, you do you.

I agree with your statement, but I will add as a reminder that second (and third, fourth, etc.) cousins are legally allowed to marry because there's almost no risk of any birth defects due to the diluted genetics from that point onwards. And if someone is marrying a third or fourth cousin, there's really zero risk of birth defects unless it's some extremely rare trait (which can happen in any setting, regardless of who you marry) or a [normal] complication during pregnancy.

:) I'd kindly mention they are not blood related. To double down on that, she is also mentally from another world. In addition, she is only seeing him again for the first time since he was sent away to boarding school -when the character, whose body she is now in, was actually killed by her step brother.
Otherwise, I agree that I am uncomfortable with blood first generation incest.. and other types but what I communicated was the law in my area so I leave it there and have a nice day

Although this story does not fall into this, but sibling dynamics can still be morally wrong- It is a power dynamic that we often do not think about and the ramifications that can have. While I get what you are saying from a physical/gene perspective but you also have to take in account the psychology of family relationships and the potential consequences.

this is actually what I'd pay more attention to. one generation of intermarriage wont actually do that much genetically, so as long as your descendants don't make a habit of it, you'd be fine. Also, that leaves the point of being in a relationship and not having children with each other.
The bigger problem is the unhealthy dynamics that get involved in a sibling relationship. In this case though, we could argue that as Yus left for boarding school, and she transmigrated in, they never saw each other as brother and sister.

I feel the same. Having a dynamic that is both romantic and sibling is uncomfortable because sibling relations carry a very similar weight to parent and child, especially when one sibling is significantly older. There are power dynamics that are helpful in the familial dynamic that are destructive in the romantic.
While it's possible for childhood friends to go from a sibling like relationship to a progression of romance, there's still a removed step of they lived in different houses, they usually spend some time apparent, and there is usually a change of circumstances (puberty?) to make at least one of the pair view the situation differently.
Here the sibling relationship doesn't even begin in the first place. The original brother step sister rarely spent time together, he went off to school, she's from another world, so I give it the ok.

Can someone point me towards an eng translation of the novel?

I don’t think there’s an English translation, but here’s the original Korean novel... https://page.kakapo.com/home/53939645
This guy is a grade A asshole. Wtf was that whole speech about “oh you’d spread your legs and suck for any one if I asked you to”
Pls go bury yourself 6 ft underground thanks