Hardened adrresses still have an extended public key it's just that it's harder (could be impossible) to find a private key given another private key of the wallet and the extended public key.
The parent extended public key is not used at all in the generation of hardened children. Hardened children are derived solely from the parent extended private key and the index. For this reason, it is indeed impossible to derive the private keys of hardened siblings by knowing a single private key and the parent extended public key.