Homeboyz by Alan Lawrence Sitomer.

When Teddy's little sister, Tina, is gunned down in a drive-by shooting, the gang members who rule the streets in the Anderson family's rapidly deteriorating neighborhood dismiss the indcident as just another case of RP, RT -- wrong place, wrong time. While the Anderson family mourns, Teddy plots revenge. He is arrested for attempted homicide and thrown in to the dark reality of California's juvenile prisons, his plans for revenge foiled. An innovative probation program earns Teddy early release from jail, and he finds himself under house arrest. As part of a juvenile rehabilitation program, Teddy is assigned to menor Micah, a twelve year old foster child and wannabe gansta who has nothing: no parents, no role models, not even a raincoat to call his own. As Teddy plots his final vengeance against the gang members who took his baby sister's life, he fails to see Micah's desperate need for love and trust. But Micah just might have the power not only to pierce Teddy's defenses, but to heal the deep wounds of the Anderson family.