I think it’s because the authors Korean so the words are directly translated from it
I can understand that, what i don’t understand is the translators keeping it when it makes no sense for the characters to use that term and when they heavily localize their translations.
probably because in english no one calls their older brother as brother so replacing hyung with "brother" wouldnt sit right so they kept it as hyung. this is a very common thing ive noticed in manhwas with western/european setting, most of the time they keep it as it is in korean instead of replacing it with english or using just the names.
I mean we don’t know their full ancestry, but it doesn’t seem like they’re half Korean, so the translation keeping (or having Brett use) “hyung” is odd.