一家人不說兩家話 [－－－－说两－话]

Jyutping
Pinyin yī jiā rén bù shuō liǎng jiā huà

Definitions (CC-CEDICT)
  1. lit. family members speak frankly with one another, not courteously, as if they were from two different families (idiom)
  2. fig. people don't need to be deferential when they ask a family member for help
  3. people from the same family should stick together (and good friends likewise)