How are ambiguous “brother” and “sister” typically translated into Chinese? 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.
I translate professionally, and teach translation as well, and I have no comfortable solution.  It is one of those cross-cultural conundrums that cannot be resolved satisfactorily with even a short equivalent, as family ranking is so precise to the Chinese, but if I must, then here's my squirm-my-way-through solution:

"Brother" and "sister" are not so bad, because at least one can translate them into Chinese as 哥哥/弟弟, 姐姐/妹妹, and hope to get away with it.  Not ideal, but doable, if clunky.

The really impossible ones are the other relatives: 2 hoops to go through for uncle / aunt (paternal side or maternal? older or younger than parent?); even more hoops to go through for cousins (male or female? paternal side or maternal? older or younger than speaker or reference person?).

What I do, if the context allows such vagueness in the Chinese and I have no way of verifying further details for the "uncle/aunt/cousin", is to go for "亲戚 qīnqi / relative" (or 亲人).  (Or turn the job down...)