After all, the chances of any two people happening to both have a non-binary nibling or pibling or sibling or parent or child is small, but the chances of any two people happening to have any sort of non-binary relative at all is considerably higher, which would hypothetically make it much easier for a unified term for “non-binary relative” to spread
Why not just use the word ‘kin’?
Well, the idea is terminology that specifically encodes being non-binary.