People who live together as a family or join a specific group often share similar temperaments, habits, or values.

他们两口子都这么爱占小便宜，真是不是一家人不进一家门。

The belief that becoming family members, especially through marriage, is a result of fate or a predestined connection.

你们俩能结为夫妻也是缘分，正所谓不是一家人不进一家门。