blood brother vs brother-in-law

blood brother

noun
  • A brother by blood: a brother-german or half-brother, as distinguished from step-brothers. 

  • An unrelated male considered to share such a strong relationship generally. 

  • An unrelated male considered to share a brother-like relationship established by a ceremonial sharing of blood in some cultures. 

brother-in-law

noun
  • The brother of the spouse of one's sibling. 

  • The brother of one's spouse. 

  • The husband of the sibling of one's spouse. 

  • The husband of one's sibling. 

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