you forgot the "big" part.
when i first saw "bigender" i thought it read "big-ender" and that this was a sexual category of people with outsize butts.
but of course you can't even say "bigender" any more – what? are you implying there are only two genders? how darest thou!
but the "big gender-neutral non-binary older sibling" is ok. except … now we're sizeist (how dare you imply that big is better) and ageist (how dare you imply that older is wiser).
have to say it, nicki minaj was my classic big-ender, but she has come through for the people. you never know.