But thats not a proper translation.
There was no "good" way around this issue, its a form of address that essentially doesn't exist in English.
You either leave it as "Onee-chan", include a TL Note, and have the audience translate it in their head each time, or just translate it, deal wiht the fact that its awkward, and move onwards.
Both strategies are equally valid, and either one will prompt bitching.
Also, "Onee-sama" means "Big sister", always. The fact that in Japanese that form of address can be used to address someone who is not technically your big sister does not change the translation.