Most people would refer to this as "defense of others."
In North Carolina, though, the relevant statute, G.S. 14-51.3, formally refers to this as "Use of force in defense of person." The statute allows a defense against criminal and civil liability for non-deadly force used "against another when and to the extent that the person reasonably believes that the conduct is necessary to defend himself or herself or another against the other's imminent use of unlawful force."
Note, however, that the statute does not allow you to invoke the defense when the victim "is a law enforcement officer ... was lawfully acting in the performance of his or her official duties and ... identified himself or herself in accordance with any applicable law." This raises the question of whether the officer in this case was "lawfully acting in the performance of his official duties" when the relative intervened. If he was, the defense would likely be unavailable.