family unit vs sibling

family unit

noun
  • A unit of people consisting of family members, distant or immediate. 

sibling

noun
  • A person who shares a parent; one's brother or sister who one shares a parent with. 

  • A node in a data structure that shares its parent with another node. 

  • The most closely related species, or one of several most closely related species when none can be determined to be more closely related. 

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