adoptive mother vs stepmother

adoptive mother

noun
  • A woman who has adopted a child, as opposed to a biological mother. 

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 adoptive mother and stepmother occurred in a corpus of books? (source: Google Ngram Viewer )