"My niece is my sister's daughter."
Translation:Mia nevino estas la filino de mia fratino.
Hm. I'm a komencanto, but if I were to throw in a guess here, what about:
Mia genevo estas la gefilo de mia gefrato.
with the intended meaning of
My [one of: either nephew or niece] is my [one of: either brother or sister]'s [one of: either son or daughter].
(using the same back-formation as
gepatroj (ge·patr·o·j) parents
gepatro (ge·patr·o) a parent [one parent - not specified whether it's a mother or a father (or some other guardian)])