Mm. Sorry it's not to your taste, but she uses unique-to-her, made-up nicknames for her brother, so when she does that, we localized them. That particular line has her call him "nii-yan", for example. Translating them helps her come off as quirky in English, the way she comes off as quirky in Japanese. If it's any consolation, literally every time she uses everybody's favorite actual word, "onii-chan", it's just transliterated as "onii-chan". Nicknames are still the weirdest hill for VN fan