Responses
It's more like both of them might actually a twin since the reason why Haesol's mother hate her just because she look a like her father. But maybe they're half-sibling with the same father, different mother too
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It would be kinda hilarious if this turn out to be a love triangle, with Yeonwoo receiving love from both siblings, but considering the usual tone of this webtoon, I suspect this would turn into drama.
I wonder if the bro realizes Yeonwoo is a guy. He acted kinda like a flirt. He naturally asked for Yeonwoo's number to keep contact, and he smoothly asked Yeonwoo out for a movie. He also put his arm around Yeonwoo's shoulder. Maybe I am reading too much into it. Maybe he is just acting friendly.
My current theory about the brother is that he is Haesol's half-brother with different father. Isn't it mentioned a few chapters back that Haesol is a product of rape and that her mother has remarried? Maybe he is looking for her now after seeing her from the online pictures of Haesol.