In sentence a, the speaker has two or more brothers. (which br | Grammar Guy
In sentence a, the speaker has two or more brothers. (which brother got married? The adjective clause tells you it is the brother in Chicago, not the one in Texas.)
In sentence b, the speaker has only one brother. (The adjective clause gives additional infrmation, but because the speaker has only one brother, the clause is not needed to identify which brother.)
