grandmother vs mother-in-law

grandmother

noun

  • A mother of someone's parent. 

  • A female ancestor or progenitor. 

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 grandmother and mother-in-law occurred in a corpus of books? (source: Google Ngram Viewer )