Is there any way to refer to someones family member, lets say brother, who's relative age I am unaware of and consequently am unable to decisively call 弟弟 or 哥哥?
Is there some sort of rule, like assume they are older?
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let me use "him" to instead of the "someone", "other one" instead of "his family member", than suppose that other one is male. ccording to what you say.you must have a good relationship with him
if he haven't introduced other one yet, you could ask him
“他是你弟弟/哥哥？ 他叫什么名字？ 我比他大吗？”
so than, you could decisively call other one "弟弟/哥哥“
unless other one is older than you, you can call "哥哥” consequently. otherwise if other one is younger than you. i think call the first name is better, or the nick name which "he" call other one. just follow him.