"The girl of my sister is my niece."
Translation:Fata surorii mele este nepoata mea.
I am confused as to why "my sister" is in plural for the romanian translation but in singular for the english? I can recognize that "sister" is in singular genitive dative, but the "mele" throws me off...
It is not in plural but in singular genitive. You are misled by "surori" that is indeed the plural of "soră" but here we have "surorii" (double i). As a side discussion Romanians debate which of "surorii mele" or "sorei mele" is better. Both forms are used.