siostry

In a sentence

• Moje siostry są bardzo serdeczne.

  My sisters are very kind.

  The word 'siostry' is used here in the plural nominative case to indicate the subject of the sentence, which are the sisters.

• Widzę dwie siostry na ulicy.

  I see two sisters on the street.

  Here, 'siostry' appears in the singular accusative case because it is the object of the action 'to see.'

• Daję książkę moim siostrom.

  I am giving a book to my sisters.

  In this case, 'siostrom' is the dative form of 'siostry,' indicating to whom the book is being given.