mother-in-law vs paternal aunt

mother-in-law

noun

  • The mother of one's spouse. 

  • A mother-in-law sandwich. 

  • A mother-in-law apartment. 

paternal aunt

noun

  • The wife of a sibling of someone’s father. 

  • A sister of someone’s father. 

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