godmother vs sister-wife

godmother

noun
  • A woman present at the christening of a baby who promises to help raise the child in a Christian manner; a female godparent who sponsors the baptism of a child. 

verb
  • To act as godmother to. 

sister-wife

noun
  • A woman who is simultaneously sister and wife to her spouse. 

  • A co-wife in general. 

  • In a polygamous marriage, a woman who is simultaneously the sister and co-wife of another. 

How often have the words godmother and sister-wife occurred in a corpus of books? (source: Google Ngram Viewer )