English does not have an honorific for use with older siblings, like it does between "Dad" and "Father", where the latter has more implied honorific, and less "familial only" implication.
We dont go "Older sister, will you help me?" We go "Hey sis, will you help me?" or worse (From a japanese perspective) we use a first name, "Hey [name of sister], will you help me?"
As such, the use of onee-sama is lost to English speakers. Likewise would be "imouto". The closest we have is "onee-san", "sister".
Regardless, "sissy" is not the right word to use. That would be more appropriate a replacement for imouto, not onee-sama, nor onee-san. "sis" would be OK for onee-san, but not for onee-sama. English lacks an appropriate term. We would instead use the first name when addressing such persons.
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But my own translation WTF came from seeing the translation subtitles for Kiki's delivery service. Early in the movie, just after she moves into the bakery, she tells her cat Jiji that unless they want to eat pancakes all the time they need to get work. Jiji mentions that he likes pancakes-- and this is where the translation gets odd; The actual dialog spoken says something to the effect of "Not everyday, everyday, everyday!" while the translated text says "I'll get fat, fat, fat!" (and the dialog was changed in the english dub to reflect the translation)
That happened several years ago, but it stuck with me. How do they get "fat" (debu) out of "everyday" (mainichi) ?? The world may never know.