The Golden Bird- who is the fox?

At the very end of the tale, we learn that the fox is actually the princess' brother.

A long while after, he went to walk one day in the wood, and the old fox met him, and besought him with tears in his eyes to kill him, and cut off his head and feet. And at last he did so, and in a moment the fox was changed into a man, and turned out to be the brother of the princess, who had been lost a great many, many years.