What was that "known and unknown thing, something never to be spoken"?

As Papa and Mr. Morrison walk up the driveway, Mr. Jamison's car pulls up. He tells them that Jim Lee Barnett died at four o'clock in the morning. TJ has a couple of broken ribs and a broken jaw but will "be all right...for now." Mr. Jamison is going to bring the Averys to town and Papa wants to go, but Mr. Morrison suggests he stay clear of the situation to avoid suspicion. Cassie is confused by this comment, until she realizes that Papa started the fire himself to keep TJ and Stacey safe. She knows that this fact is "something never to be spoken, not even to each other."

hi