yī​jiā​rén​bù​shuō​liǎng​jiā​huà

frankness

一家人不说两家话 (一家人不說兩家話) yī​jiā​rén​bù​shuō​liǎng​jiā​huà
frankness
family members or close friends should speak directly and avoid unnecessary politeness or formality with one another
solidarity
people who are on the same side or from the same group should stand together and support one another

Sources

Definitions
CC-CEDICT