This is the 1716th most frequent Polish word.
siostry
"Siostry" means "sisters" in English.
The word 'siostry' is used here in the plural nominative case to indicate the subject of the sentence, which are the sisters.
Moje siostry są bardzo serdeczne.
My sisters are very kind.
Here, 'siostry' appears in the singular accusative case because it is the object of the action 'to see.'
Widzę dwie siostry na ulicy.
I see two sisters on the street.
In this case, 'siostrom' is the dative form of 'siostry,' indicating to whom the book is being given.