-My brother who lives in spain is visiting next week.
-My brother, who lives in spain, is visiting next week.
The first, without the comma after 'brother', is trying to suggest this is just one of his brothers - the one who lives in Spain.
The second might be trying to suggest that he has only one brother, (who incidentally, lives in Spain.)
Good try, but in real life, I still think people would be unsure!