Your sister needs to have her 15-year-old son evaluated by a professional. When a teenager demonstrates violent and threatening behavior, a parent needs to react quickly before someone gets hurt, including the teenager himself. Where is he getting his alcohol? Is the person providing it to any other minors? This is illegal, and it should be promptly dealt with as well.
Boys Town has a database for the entire United States, and we would gladly provide referrals to you or your sister. You can either e-mail us back with the city, state and county where she lives, or you can call our Hotline for the information. Most mental health centers can do an evaluation and make recommendations for further counseling. We have crisis counselors available to talk 24/7. Please give her our phone number: 1-800-448-3000.