Translation:My older sister is outside.
I wrote "Sister is outside" because it wasn't clear whether it was saying "my sister" or "your sister". This sort of phrase in Japanese is clearly context dependent. This answer should be accepted right? Or is there another word that I didn't know about that is used to refer to someone else's sister?
I'm sort of confused about your last sentence. Do you think that "Sister is outside" refers to someone else's sister or your own? To me it could only be the later, and then only rather idiosyncratically.
In any case, it seems the word here very much refers to one's own older sister: https://en.wiktionary.org/wiki/%E3%81%82%E3%81%AD#Japanese