"Žofie je dcera mé sestřenice Kateřiny."

Translation: Žofie is the daughter of my cousin Kateřina.

[LongFellow3]

I translated sestrenice as niece, that is another word for cousin but apparently it was not added to the list of accepted answers. Instead I got the remark that i used "a wrong word" ...lol

[JanLyko]

In what universe?

NIECE

a daughter of your brother or sister, or a daughter of your husband's or wife's brother or sister (Cambridge English Dictionary)

a daughter of one's brother or sister, or of one's brother-in-law or sister-in-law (Oxford British & World English Dictionary)

a daughter of one's brother, sister, brother-in-law, or sister-in-law (Merriam-Webster)

[Hatul_Madan]

Niece is siostrzenica in Polish.

[ion1122]

In English, "cousin" and "niece" are two different words with two different meanings. The Czech word for "niece" is "neteř".