How even

technically it is incest? even though they’re not blood related they’re still siblings which make it incest

It's pseudo incest.

due to legal clause, incest NEEDS blood relation at the first line of degree such as: blood-father with blood-son, blood-grandfather with blood-niece and blood-brothers. stop. and literally they were old enough when she was adopted, it's a fundamental thing because they didn't grow up together. He knew at first she was not his sister, she is "more like sister" but she isn't. it's a different situation in my opinion but it could annoy someone, it's legittimate

Exactly!!! Thas how I see it

I'm sure she is returned to the past because something in her relationship with Eugene went wrong but she hasn't realized yet. She remember something about past life, moments with Eugene in which he talked to her with kind of sad expression. I think he has questioned himself several times about if it is right or wrong, she is "sister" but she is not: do he worth enough to be happy with her? there are others guys ready to take care of her, he could go on with another girl, good girl, maybe he'd find not happiness but comfort-- I pity this kind of past thoughts of him.
However, it's complicated and this story has to be read without prejudices or go on, it isn't story for you if it makes you unconfortable ╮(￣▽￣)╭

usually step siblings or adopted siblings doesnt bother me as it isnt incest, but the fact that she looks basically exactly like their REAL sister is what makes it weird for me.

yeah, this is a particular fact-- I think it's a way to talk about the mother's trauma explaining her bad condition but basically they are two different people, two different girls with similar physical characteristics

True , but they said only thing that’s the same is the hair color & that she was around the same age that’s the only reason tbh

You seem to forget, she looks like his fucking real blood sister. That's creept af

This!! I totally agree!