Translation:Žofie is the daughter of my cousin Kateřina.
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
In what universe?
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)