brother-in-law vs relative-in-law

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. 

relative-in-law

noun
  • Co-relative-in-law: A relative of one's generation, separated by two degrees of marriage. 

  • A relative of one's generation, separated by one degree of marriage. 

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