Like several other languages, Standard Chinese has different words for "older brother" (哥哥), "younger brother" (弟弟), "older sister" (姐姐) and "younger sister" (妹妹). However, many languages don't express this type of age relationship by means of nouns only, but by means of optional adjectives. Examples include English, German, French and Dutch.
So when translating words such as "brother" and "sister" (in the singular form!) in texts from the latter category of languages into Chinese and when there is no context that indicates the age difference, what do Chinese translators do? As far as I know, words such as 兄弟 ("brother") and 兄弟姐妹 ("brothers and sisters")) aren't used to refer to a single sibling and wouldn't be correct as translation for, e.g., (singular) "brother".
Note: I hope it is clear that this is not a simple "translate-this-for-me" question, but a question of how translators deal with this.