What should I do about my brother who raises his voice at my mother and hurts her with words, and we cannot tell our father because my brother pretends to be obedient in front of him? Should I complain to my father or suffice with offering advice and being patient with him?

Your brother has committed one of the greatest major sins, which is disobedience to his mother. Therefore, you must advise him and clarify the religious texts to him, and threaten to escalate the matter to your father if he does not stop. If he does not desist, then escalate the matter to your father so that he may stop him, and explain to your mother that the complaint is for his benefit so that he does not persist in disobedience, and that excessive mercy may be harmful.