I think if it’s spoken in the game, then it should also be included in the text. That’s just me though.
100% agree. The game has a Japanese source. The equivalent in English is jarring to the point it breaks the immersion, whereas in Japanese this is very natural. Either the line needs a rewrite to get close to the original meaning, or it should simply be translated “oniichan” since the intended audience knows exactly what is meant by it.