translation - How are ambiguous "brother" and "sister" typically translated into Japanese?

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.

Answer

Think, what pronoun would you typically use for a person whose sex is completely unknown (in Japanese, genderless pronouns are more commonly used)? This is a very similar question.

The Japanese translator of Harry Potter series went to ask J.K. Rowling if a character is another one's older sister or younger sister. This is always by far the best practice than anything written below. (But unfortunately it's after the first volume was published, so in JP version someone's siblinghood is twisted in the middle of the series.)

In some occasions you could use 兄弟 or 姉妹. But these words never mean a singular sibling, always "brothers" and "sisters". They aren't made for describing interpersonal relationship between two people, but their common property.

彼女はルーシーの姉妹だ

It would likely to mean "she is one of Lucy's female siblings", or more practically, "she was born from the same parents with Lucy" or "she shares gene with Lucy" etc., that could be said during profiling or search of bone marrow donors. (Or they belong to a "sorority", or are mass production humanoids etc. etc.)

However, there's a workaround:

彼女とルーシーは姉妹だ

can successfully tell their relationship without raising much eyebrows. Of course, it's useless when someone is referred only by their relation, e.g. "My sister is..."

You can also avoid to mention relationship terms when it's possible, as one of common solutions translators take to cope with untranslatable words.

The ultimate option is to take a shot in the dark. Older or younger, fifty-fifty :) When you mention a fictional character and they are disposable in the plot, this works great, but not recommendable for a real person. I don't know how many minor characters in drama were hushed up like this.