NVC will not expedite a numerically limited visa (for which there is no visa number available yet), for any reason, including medical.
If she is already receiving treatment, why would she need to come to the US for treatment?
The only option would be a B visa for medical, but with your ties to the US, getting that is unlikely.
And how do you plan to pay for it? You will need the cash up front, since you won’t have insurance. Medical care and drugs in the United States are extremely expensive. Infusions of immunotherapy drugs for autoimmune disorders like Lupus can cost $20k per dose!
Sorry, I think you’ll have to wait for your immigrant visa PD to become current.