From the Sixt T&C:

Cross Border Rentals & Territorial Restrictions
Cross Border Rentals are allowed to the following countries:
Belgium, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Great Britain, Germany, Hungary, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Monaco, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden and Switzerland.

Cross boarder rentals with Audi, BMW, Mercedes, VW cars as well as Jeeps/Offroader are not allowed to the following countries: Croatia, Czech Republic, Estonia, Hungary, Latvia, Lithunia, Poland, Slovakia and Slovenia.
It is also allowed to enter the last-mentioned countries with all vehicles of car groups IVMR, SVMR and FVMR (including all mercedes) and vans.


So if they only rent BMW at this station (what I personally cannot imagine) it will not work...

Greez,
Jonathan