家丑不可外扬 Family shame must not be spread outside. The private failings of a family are not to be made known to outsiders. The Handian dictionary cites an earlier form, 家丑不外扬, from the Song dynasty Compendium of Five Lamps (五灯会元).
一家人不说两家话 One family does not speak the words of two families. Among one’s own people, there is no need for polite formulas.
体己话 Intimate words. The private talk between people who are close. The Handian dictionary takes its example from Dream of the Red Chamber.
知根知底 To know the root and the bottom. To know someone well. The dictionary’s example reads: “We are old friends. We know each other through and through.”