How do I refer to someone's family member who's relative age I am unsure of? 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?
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


  小明跟小華是兄弟