niece vs stepsister

niece

noun
  • A daughter of one’s sibling, brother-in-law, or sister-in-law; either the daughter of one's brother ("fraternal niece"), or of one's sister ("sororal niece"). 

  • A daughter of one’s cousin or cousin-in-law 

stepsister

noun
  • The stepdaughter of one's parent which is not one's half-sister. 

  • The daughter of one's stepparent who is not the daughter of either of one's parents. 

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