Update: I messaged Sixt on FB, and they told me to email the Customer service dept. Here is their reply,
"Dear Mr. ...,
At Sixt service is our ultimate priority. This is why we are always there for you if you have any questions.
We cancelled the reservation and waived the cancellation fee and credit the relevant amount to the credit card account.
If there is anything else we can do for you at any time, please contact us. We are always at your service.
Get well soon Mr. ..'
Best regards"