This is the 15259th most frequent Polish word.
siostrom
"Siostrom" means "to (the) sisters" in English.
Here, 'siostrom' is in the dative case, indicating the indirect object, the sisters, who are receiving the books.
Podarowałem książki siostrom.
I gave books to the sisters.
In this sentence, 'siostrom' is in the dative case, showing the recipient of the gratitude, the sisters.
Siostrom zawsze dziękuję za pomoc.
I always thank the sisters for their help.
The word 'siostrom' is in the dative case, representing the indirect object, the sisters, to whom the rules are being explained.