Considering Sixt have the largest BMW fleet in the world, you could probably be 99% confident of getting a BMW in Germany if you aren't too picky about which model you'll get.
In my experience, the BMW 1er isn't common in Germany as an automatic. I once tried booking it on their website as a CPAR and the large downtown Berlin location did not even have any automatic cars in that class. However, if you get FDAR or above, you'll certainly be able to choose a BMW. In particular, I've never seen station wagons at Sixt that are not the BMW 3 or 5 series.
Another problem is that Freiburg only has one Sixt station. If you really want a 1er automatic, you could try calling the Mulhouse and Basel locations on the day that you're renting.
If you 100% want a specific BMW model, you can try reserving a Hertz Prestige Collection vehicle.