It has been raised here a few times in the past that Sixt did not include winter tires by default but was asking for an extra fee. This was considered to be extra-sneaky as German law mandates winter tires if the circumstances make those necessary.

In the end it came to a gamble whether one pays the extra fee and is guaranteed a car with winter tires or if one hopes that the car one gets already has winter tires.

Sixt has now announced that winter tires will be included in the rental fees during winter season. Link in German is at: http://www.sixtblog.de/pressemeldung...all-inclusive/.
The blog post talks about reduced rental fees if the weather conditions allow the rental of a car with summer tires. I guess this means that actual rentals will incur higher rental fees. But nevertheless, I guess this is an improvement. Even if it is not really "for free".

I have a reservation from Dec 5 through Dec 9. I made it without asking for winter tires (which, if I remember correctly, would have cost me about 18 Euros more per day).

What happens now? Will Sixt honor my original price but give me winter tires for free? Will they charge me extra for those tires?

You will be charged according to your reservation, if you have the need for winter tyres then this would be charged extra.

You could of course rebook with winter tyres inclusive but then you would have a new reservation with new prices. This wouldn't be a good option if you booked a pre paid rental as in that case a cancellation fee would be charged.

Gary - Customer Service Team