After reading all the posts here I think the worse thing you could do is leave. That would only give the bully more leverage to act even worse. From what you have written, this kid is screaming out for help. Sit down and talk to hubby, make it clear that if he doesn't act on this you will, tell him "NO MORE." This kid will only get worse and is big enough to cause permanent damage to a sibling. It's obvious that the break up of his parents has left emotional scars and he is acting out in anger and frustration. Sit all the kids down and tell them exactly what the rules are and if there is any further abuse from the 15 yr old the police will be called and charges laid, then DO IT.
You are not doing the kids a favor by running away to escape all this. The whole family should be getting counselling to become a real family. If no one wants to go, then go by yourself and reach out for help.
The first step is calling the police and laying charges...the crown will prosecute the boy, might send him to a juvenile center, which is what he and the family needs right now before it becomes irreversible .