admit vs let someone in on something

admit

verb
  • To allow to enter; to grant entrance (to), whether into a place, into the mind, or into consideration 

  • To give warrant or allowance, to grant opportunity or permission (+ of). 

  • To allow (someone) to enter a profession or to enjoy a privilege; to recognize as qualified for a franchise. 

  • To concede as true; to acknowledge or assent to, as an allegation which it is impossible to deny (+ to). 

  • To allow to enter a hospital or similar facility for treatment. 

  • To be capable of; to permit. In this sense, "of" may be used after the verb, or may be omitted. 

let someone in on something

How often have the words admit and let someone in on something occurred in a corpus of books? (source: Google Ngram Viewer )