"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)])
You will be understood, but be aware that using "ge-" in the singular isn't widely accepted yet.
This sounds like it could be a song or a poem. Every -o ending is preceded by the same number of syllables.
Because that would be "My niece is my sister-daughter." It brings to mind Sister Wives, at least to my ear. ;-) The -a ending is an adjective, not a possessive.
Two reasons: First, you can't say "sister daughter", you need to say "daughter of my sister". You're missing the possessive part.
Second, "esti" is a stative verb, which by its nature is not a transitive verb. You shouldn't have the accusative -n endings.