It's becoming a bit delicate to translate the adjective hezky: depending on the sentence, it could be "nice", "cute", "pretty" etc but in a very arbitrary manner. "My cousin has a cute blue skirt" was not accepted but I imagine one could say that?

sestřenice is both singular and plural?

Another possible translation from the English to the Czech:
Můj bratranec má hezkou modrou sukni.