@cumdumpster1 Not every people know what oniichan means, because if i follow your logic in a coreen manga, every reader have to know what oppa or hyung stand for....
And also, "There are certain words you should never translate" i translate the term "akka" (means "older sister" in tamil, an indian language) to "grand soeur" (also means older sister in french) when i call out my older sister... So when I saw the term "big brother", it doesn't bother me because i speak like that irl.
It's just your occidental bias and your japanese's knowledge who put you in case where you consider this is is common sense but it not...
A reader doesn't have to know the original language for being able to comfortably enjoy an translation.
If a reader have to search for the meaning of a word then, the translator fails to do his job properly.^^