lit. family shames must not be spread abroad (idiom)
fig. don't wash your dirty linen in public
Definitions (粵典–words.hk)
語句
指屋企(或者引申到學校、公司、組織等)入面嘅醜事,唔應該俾外人知道,應該私下處理 bad things about one's family (or school, company, organisation, etc.) should not be aired to outsiders, and should be settled within the circle
參看
家醜不出外揚、家醜不出外傳、家醜不可外傳、家醜不得外傳
Definitions (Wiktionary)
proverb
what happens in the family stays in the family; don't wash your dirty linen in public