We were helped by excellent front desk staff, with great English, and got a clean and functional car. We would be happy to rent again from here if we end up in Sintra again.
One thing to be aware of: the location shows up correctly in Google maps. However, there is almost no signage outside, as they lease a bit of space from a Renault dealership. Just go into the dealership and ask for Sixt (forward and to the left from the main entrance).