half sister vs second cousin

half sister

noun
  • A female sibling sharing a single parent. 

second cousin

noun
  • A grandchild of a grandparent's sibling. 

  • A relative who has with the other person only two great-grandparents in common, but where the parents and grandparents are different. 

How often have the words half sister and second cousin occurred in a corpus of books? (source: Google Ngram Viewer )