How are ambiguous “brother” and “sister” typically translated into Chinese?
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...)