Is a brother who curses or beats his sister for not obeying him in a sin against Allah considered one of the warring disbelievers whom one must abandon?

Striking or reviling one's brother for not obeying him in an act of disobedience to Allah does not make him a disbelieving combatant who must be shunned; rather, he is a sinful Muslim. Shunning a disobedient person is a means of deterring and restraining, and in it, the benefits and harms should be weighed.