I showed this question to a Chinese friend whose non-binary-ish. They said they didn't know, then asked their sister how to answer this question. Their sister said she just uses their name. So you could be forgiven for not knowing.
If it were me, I'd just use the English word "sibling" when speaking Chinese, along the lines of how the English word "gay" is used in Chinese (他是gay吗？). Such as:
I have a non-binary sibling.
It's possible to explain that Chinese doesn't (yet) really have a word for this concept.
Chinese doesn't have a completely suitable word. I just call them [...].
When it comes down to it, Chinese simply doesn't have the word "sibling"; 兄弟姐妹 ("older/younger brother, older/younger sister") is the gendered language that we probably want to avoid in this context.
That being said, non-binary people are acutely aware of these language complications, and some are okay with gendered language (maybe for practical reasons, or maybe they're closer to e.g. the masculine end of the spectrum). But this varies from person to person---they'd have to tell you.