be there vs sit in on

be there

verb
  • To be available to provide comfort and support for someone, especially in a period of difficulty. 

  • Used other than figuratively or idiomatically: see be, there. 

sit in on

verb
  • To attend a private meeting or class without participating. 

How often have the words be there and sit in on occurred in a corpus of books? (source: Google Ngram Viewer )