amma vs stepmother

amma

noun
  • mother 

  • An abbess or spiritual mother, especially one of the Desert Mothers. 

stepmother

noun
  • The wife of one's biological father, other than one's biological mother. 

  • A viola, especially Viola tricolor, heartsease. 

How often have the words amma and stepmother occurred in a corpus of books? (source: Google Ngram Viewer )