co-father vs stepmother

co-father

noun
  • In a male homosexual couple, the nonbiological father (partner of the biological father) of a child. 

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 co-father and stepmother occurred in a corpus of books? (source: Google Ngram Viewer )