That's their culture. It's not like the author used the same word because they couldn't come up with a plot.
lmao don't. it doesn't make it incest-ish(?). you don't only call a lover or relative some way, it is used to everyone and tbh doesn't always mean anything other than an age-based relationship. there are other korean words for your partner and here it's just being used for the plot
most countries in asia use honorifics
Well, to be fair in English there is a similar thing with "sweetheart" which can be used for both one's lover and one's children (especially if they're female children). If it makes it any better, avoid translating it as "brother" in your head, and then it won't sound that weird... (besides, it's not like it's the exact translation either).
Seriously... It makes 0 sense to use the same term for brother and boyfriend... Makes it all incesty sounding and weird.. I'm interested in this but I feel like it could've been solved around chapter 20 and done with. This is being stretched out so much to where it's stupid in all honesty..