"Brethren" is old fashioned. It is only used by religious or spiritual groups that consider themselves something "special" and above the rest of humanity. LOL. This is true. Imagine the Moonies. They might call each other "brethren".

Nowadays, no one uses the word 'brethren' unless they are talking about a religious organisation. "Professional colleagues" or "co-workers" is more appropriate to describe people whom you work with.

"Brethren" is not normally used in conversation. It has more a religious connotation.