I have long given up on expecting any sort of consistency with a lot of things when it comes to fan translations. You know, only certain characters using honorifics, inserting romaji in place of the actual English equivalent (bit more jarring when one uses elder sister and onii-chan in the same translation), or 95% of any Japanese upload using a romaji title even if the author includes the English title.
But Onī-chan?
If you're translating to English, this isn't a thing. Your English seems good otherwise but this is weird enough that I felt compelled to complain.
Go with big brother or onii-chan if you want to convey the informality without localizing to big bro.
@OlgaFrow I appreciate your feedback and hear you on that awkwardness! The full localization to "big bro" does sound strange to me personally, hence the "Onī-chan," but I understand how writing it that way can make for an inconsistent reading experience. I'll try and apply this to anything I work on in the future. Thanks again!