Japanese has different words for "younger sibling" and "older sibling". Then how are the English words "brother" and "sister" (or any other European equivalents for that matter) typically translated into Japanese (in books, TV, movies, etc.) without a clear context or clue as to how older or younger the sibling in question is (which is the case most of the time: Western authors don't bother to use "younger/older sibling", or give any clue, at all)? For example, how do Japanese translators deal with a vague phrase such as "Lucy's sister"? Do they just randomly use either 「ルーシーの姉」 or 「ルーシーの妹」? Also, what about the case of twin siblings?
Note: I'm not asking can you translate these words for me?, but how are these words typically translated? based on the most common practice used by translators; i.e. I'm asking for how translation works in these specific cases in general, not for specific translations in any given contexts. So I don't think the question is off-topic.