rodzinie

In a sentence

• Mówiłem o mojej rodzinie w szkole.

  I talked about my family at school.

  This usage indicates the dative case of the noun to indicate the object of the preposition 'o'.

• Chciałbym pomóc w rodzinie mojej siostry.

  I would like to help in my sister's family.

  The prepositional case indicates location or association with 'w'.

• Od dawna nie rozmawiałem z ciocią o rodzinie.

  I haven't talked to my aunt about the family in a long time.

  Here, the dative case emphasizes the topic of conversation involving the preposition 'o'.