parent-in-law vs relative-in-law

parent-in-law

noun

  • One's spouse's parent. 

relative-in-law

noun

  • A relative of one's generation, separated by one degree of marriage. 

  • Co-relative-in-law: A relative of one's generation, separated by two degrees of marriage. 

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