Brother vs brother-in-arms

Brother

noun
  • 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. 

  • 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. 

brother-in-arms

noun
  • comrade in arms (fellow combatant or soldier) 

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