aunt-in-law vs mother-in-law

aunt-in-law

noun

  • The aunt of one's husband. 

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

  • The aunt of one's wife. 

mother-in-law

noun

  • The mother of one's spouse. 

  • A mother-in-law sandwich. 

  • A mother-in-law apartment. 

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