My brother, who lives in Barcelona, is rich.
My brother who lives in Barcelona is rich.
In which of these phrases does the speaker have more than one brother?
In the second one.
This is the case when comma defines the meaning.
“who lives in Barcelona” is a relative clause.
If this is a piece of additional information, which can be easily omitted without changing the meaning, then it´s a non-defining relative clause. And we use a comma for it.
Jack, who’s retired now, spends a lot of time with his grandchildren.
We want to see the new Tom Carter film, which was released on Friday.
My sister, whose dog I’m looking after, is visiting a friend in Australia.
Now, if this piece of information defines the noun, we call it a defining clause and don´t use a comma for it.
“My brother who finished university this summer is spending the year travelling.” (The speaker has more than one brother)
“The house whose doors are shuttered is rumoured to be haunted!” (The speaker refers to that very house from many on the street)