Do you have to wear crash helmet on motorbike with sidecar?

You don't need to wear a crash helmet by law ,however, it as seen as safer.

I'm afraid I have to disagree with the above answer, at least as far as the UK is concerned.

The position in the UK is that the rider and the pillion passenger on a motorcycle with an attached sidecar MUST wear a helmet. However, the passenger seated in the side car does not.

The driver or passengers in a three wheeled car car are not required to wear helmets.

The legal position with trikes is more complicated and advice should be taken from the local taxation office on which class the vehicle is registered in. A non legal opinion is that it depends on whether you sit astride or in the trike.

Head injuries can be fatal and are often very serious so wearing a helmet is a sensible precaution where head injuries are possible and that would include most passengers in sidecars.