"My niece is my sister's daughter."

Translation: Mia nevino estas la filino de mia fratino.

[picsa]

How could I refer to a gender variant relative in Eo?

[phle70]

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)])

[danielqsc]

You will be understood, but be aware that using "ge-" in the singular isn't widely accepted yet.

[Ariaflame]

Well, Esperanto is a hackable language. So the more it is used, the more accepted it will be.

[SariniLynn]

Would idino also mean daughter? Female offspring?

[Rae.F]

From what I understand, "ido" is not preferred for use with human beings.

[JackBond]

This sounds like it could be a song or a poem. Every -o ending is preceded by the same number of syllables.

[Evannex]

Awe... Why is "Mia nevino estas mian fratinan filinon." not correct? :(

[SariniLynn]

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.

[Evannex]

Oh i see! Thank you!

[Rae.F]

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.

[HuggiePuggie]

is this wrong ?- Mia nevino estas mia fratino filino

[Rae.F]

Yes, it's wrong. You don't have any possession there, just two nouns in a row. You can't say "sister daughter", you need to say "daughter of my sister".