Your brother is only 14 years old, he is an adolescent; there are many reasons to show such a behaviour. Tantrums are a normal part of child development. They are the way young children show they're upset or frustrated. Tantrums may happen when your brother is tired, hungry, or uncomfortable; or because he can't get or do something he want. He can be self-centered and may be having too much of expectations. Also there could be pressure in academics from parents and/or from teachers OR his status of peer relationship in school that impacts and triggers anger. More than being a sibling, give him the status of a good friend to him, take him out and talk to him by offering help and NOT by advising. Let him talk with your openly; for which you must give him that confidence. Take him to a Counsellor in your city for further help, if his behaviour still persists.