godsib vs stepmother

godsib

noun
  • One's sibling or kin via a godfamily tie: one's or one's child's godparent, or one's godchild's parent, or one's godparent's 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 godsib and stepmother occurred in a corpus of books? (source: Google Ngram Viewer )