This is a tough issue for sure. You want to get help but don't want him sent away to a Juevenial Home or Detention Center. I recommend you first keep them separated to minimize any damage to either, but also determine if this is an ongoing threat or if it has really stopped. If it has stopped, I recommend that you go to a psychologist yourself first to discuss the issue of what to do (without the kids). Be open with them about your concerns and get help for your son; ask the Doctor what they would recommend. Generally speaking, psychologists have a duty to report activity that they know or think will be illegal in the future that would harm another person. If it is just past harm, that is what they help with every day though again, I'm not licensed in NY and recommend you talk with a non-profit lawyer there to double check.