Well, the nobility and many royal families were established through the use of force throughout the World history, so this is nothing out of the ordinary.
the soldier did try to steal a horse and attacked them with his sword and tried to murder them... personaly i would hang anyone siding with that soldier turned bandit for treason and bandiry.
To stand up against nobles, they would become nobles in a clan themselves. They already have the castle. I suppose this defense will give them political legitimacy.