I do have to agree with
@Marchimarch in english you aren’t going to call anyone brother, sister, uncle, etc unless they’re your actual family members (or your in a cult; I guess if you’re black too, but I’m black and that’s both never made sense or sounded right to me). It would be best to just leave it as Onee-san. A direct translation here messes with the context and connotation. That is to say no one in an English speaking country that I know of will walk up to some random older woman and call them “big sister” without getting some strange looks or everyone thinking they’re actually related (maybe close friends and even if a community uses sister or brother they aren’t going to specify older or younger). The most polite we’d call them is sir ma’am/madam (which also makes some people mad because it suggests that person is old...er; being polite is unnecessarily difficult in the U.S. if that gives any sense to it). Honestly, though you’re generally just going to shout “hey” and/or just immediately start talking in the general direction of the stranger while looking at them (yeah, we’re not a polite lot; if it says anything polite american (will hold the door open for you and say have a nice day among other stuff) is also historically racist america....). Then when you get to know them, you just use their name any time you want to refer to them specifically, and that’s it. Unlike, Japanese English doesn’t use honorifics or have any special stipulation (culturally) about using first names. That’s all just to say it hits WAY different when you translate Onee-san directly because english is culturally a different universe and the word just has a different connotative meaning.... I probably went on about that too much.