Wait. Is this going to a different direction than the novel? Or am I missing something? I'...

It seems like his sister also has the illness. That's why he has to choose. The hint was shown when Beru said "all she does is sleep."

Spoilers
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I actually am also confused cuz in the novel I don’t recall her having the illness either, in the end he just didn’t help him until later when he was already asleep and I think it was because of his friend or something... the manga might go in a different direction but the last panels are confusing because it implies some foreshadowing that he can only save on of them when I don’t remember this being a super major plot point. I might be incorrect or remembering it wrong tho

I also didn't read this in the novel I guess the MAnhwa wants to change or make some plot twist here and there

Yeah she didn't hv dat illness in novel u r not wrong mate

Exactly. That is what I'm getting as well from the last part that he has to choose. Now, I just don't remember how many "potions" he have left. Coz I think he made 3?
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I just hope it won't change much from the novel. Because I believe hin saving Jinhoo's father is an important part of the story.

Indeed it is at this point I don't know anymore hope they don't change to much

Umm if I may ask do you have the link of the novel cuz I really want to read it