Definition
Of a secret, trait, business, or a tradition that is kept within a family, shared among family members, and not disclosed to those outside the family.
Of a group of people who share similar characteristics, values, or backgrounds.
How People Actually Use It
One of the best sitcoms ever made. About Archie Bunker, Edith Bunker, Mike Stivic, and Gloria Stivic.
"All in the family is awesome"
Source: Urban Dictionary