How do you deal with a cruel child?

my nephew, justin, is living with my family because his parents are splitting up. his older brother, billy, is in the army. justin had always been a wild kid, but his behaviour is getting really out of hand. there is a little girl in my neighbourhood called saffy, who had survived leukemia, lung cancer, and a brain tremour. she had many doses of chemotherapy, operations, and undergone many other treatments. saffy also has heart and kidney problems. she has vision impairment and hearing loss as well. saffy had also lost her arms, legs and ears to a bacteria infection. justin and his gang had been harring her. they bully saffy both physically and verbally. justin and his friends had strangled her, knocked her out of her wheelchair, spat on her, threw rocks at her, and attacked her with a club. as a responsible aunt, what should i do?