fraternal niece vs stepsister

fraternal niece

noun
  • The daughter of one's brother. 

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 fraternal niece and stepsister occurred in a corpus of books? (source: Google Ngram Viewer )