Buddhism was never intended to be dogmatic,but followers of Buddha were given the freedom to use their life experience and reason to find which practice were best,and pursue those.It could be argued that it is a great act of compassion to defend another,in this case the boy who wants to protect his sister and family from physical harm, especially knowing you would be taking physical harm yourself in order to do so, and the Buddha thought compassion was one of our most important virtues.Certainly self defense and the defense of others ,would be different from acting out of frustration,or anger,or going on the offensive in some way.