Why does my brother do this?

This is serious. I think you should take him to the doctor. You might never know what might happen next time because your brother might hurt himself when hitting his head as he might hit it against something really hard, or maybe he might fall off the stairs and such because there's possibilities of it happening. Does your brother know about what he does during his sleep? Do you know when this started and for how long? You seriously need to take him to the doctors and this is for his own's sake and safety. Maybe your brother is going through a rough time and so he should never be left like this because later in life it might be too late. Therefore I strongly recommend you take him first to a doctor and then to a specialist such as a psychiatrist etc .. Wish him the very best :)