custodian vs foster mother

custodian

noun
  • A person entrusted with the custody or care of something or someone; a caretaker or keeper. 

  • a janitor; a cleaner 

foster mother

noun
  • A female adult who cares for a child who has been placed in foster care. 

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