一家人不说两家话

yī jiā rén bù shuō liǎng jiā huàyijiarenbushuoliangjiahua

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)

Study it in Yanamino