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?
There's no rule in a such situation. You can always ask politely or figure out the relationship from conversations.
But there's no harm in assuming they're younger than you or the person introducing you and that family member, especially that relative is a female ;)
You could use the terms 兄弟 and 姐妹.
For example, lets say you know 小明 and 小華 are brothers but don't know who is older. You could then say
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.