Sounds like the 16 year old needs mental health care, if there is a safety issue and aggression involved. I do not know the internal happenings inside the family, but wonder if possible Bipolar Disorder or substance abuse problem.
The family can refer to CABF foundation online for information on adolescents and BP dx.

hmmm, or, the older bro' routinely pounds on lil' bro and little bro' just got sick and tired and didn't want to take it anymore,(I am the youngest of four, it's a perspective...) or maybe they have two different dads and big bro' is the favorite... or maybe mom has few parenting skills and doesn't know how to manage two boys....okay I have to stop now, the possibilities are endless, be careful how you evaluate domestic issues when you don't know the internal workings of the family. I'm just sayin'.

Kate, true I do not know the internal workings of the family, the possibilities are endless. I am the mother of a son with BP1 and extreme rage and aggression goes with the territory. But I may be way off base, but wanted to throw it out there.