mother-in-law vs parent-in-law

mother-in-law

noun

  • The mother of one's spouse. 

  • A mother-in-law sandwich. 

  • A mother-in-law apartment. 

parent-in-law

noun

  • One's spouse's parent. 

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