aunt-in-law vs co-sister-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-sister-in-law

noun
  • One's spouse's sister-in-law, especially one's husband's brother's wife, one's brother's wife in relation to the spouses of his siblings; either of two (or more) women who are married to brothers, in relation to the other. 

  • One's brother-in-law or sister-in-law's sister; one's sibling's spouse's sister; either of two (or more) women whose siblings are married, in relation to the other. 

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