aunt-in-law vs co-brother-in-law

aunt-in-law

noun
  • The wife of one's uncle or aunt (parent's sibling). 

  • The aunt of one's husband. 

  • The aunt of one's wife. 

co-brother-in-law

noun
  • One's brother-in-law's or sister-in-law's brother; that is, one's sibling's spouse's brother; either of two (or more) men whose siblings are married to each other. 

  • One's spouse's brother-in-law (one's spouse's sibling's husband), especially one's wife's sister's husband; either of two (or more) men who marry sisters, in relation to the other; the brother of one spouse in relation to the siblings of the other spouse. 

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