best man vs brother-in-law

best man

noun
  • The primary attendant to the groom in a wedding ceremony. 

  • Used other than figuratively or idiomatically: see best, man. 

brother-in-law

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

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

  • The brother of one's spouse. 

  • The husband of one's sibling. 

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