As of today, Airberlin has posted on its website that you can carry small instruments, apparently including violins, free of charge as cabin baggage.
Here’s the wording:
Following registration, musical instruments with maximum dimensions of 86 cm x 33 cm x 19 cm (including instrument case and/or bag) and a maximum weight of 8 kg may be transported in the aircraft cabin in place of hand baggage. For larger instruments, it is mandatory to book an extra seat through our Service Centre!
Here’s the link in case you need to show it to a recalcitrant check-in agent.
They used to be the worst of airlines. Let’s see if they mean what they say this time. (They have gone back on their pledge before.)