half sister vs stepsister

half sister

noun
  • A female sibling sharing a single parent. 

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