Brother vs brethren

Brother

noun
  • A title used to personify or respectfully refer to concepts or animals. 

  • Title of respect for an adult male member of a religious or fraternal order. 

  • Title of respect for an adult male member of a fraternal/sororal organization, or comrade in a movement, or even a stranger using fictive kin. 

brethren

noun
  • kinsmen 

  • The body of members, especially of a fraternal, religious or military order. 

adj
  • Of or akin to; related; like 

How often have the words Brother and brethren occurred in a corpus of books? (source: Google Ngram Viewer )