I am so very sorry.  You are numb with shock and pain as I was 18 months ago when I experienced a similar situation with my then 17 year old adopted daughter. 

She is a predator who has since been diagnosed with anti-social personality disorder and attachment disorder.  Despite countless hours of therapy prior to and during her incarceration, she will never be cured.  She is what she is....but like your son, she had a lot of people snowed for a long time.

     The courts are hesitant to prosecute your son not only because of his age but also because it is a "family matter" and he has no previous convictions.  Juvenile corrections is about rehabilitation and getting the child back into the family unit.  You now recognize that this can never be.  However, you may have a very long and hard time getting others on the same page...especially since he is so young.

     Whether he goes to a group home or prison is immaterial.  Both are restricted environments where he will be watched and evaluated and his behavior documented.  If you have to go to therapy with him, I recommend a RAD therapist.  They specialize in attachment disorder and have pretty much seen it all.  They are not easily snowed.

     Even though it may feel like your world is caving in.....please take solace in the fact that since it is all out in the open, the healing can begin.  Stay strong for the girls to the degree that you can.  They will soon see that you will go any length to keep them safe.  If you are feeling guilty...try not to.  The fact that he got away with it so long, shows how clever and manipulative he is.  Don't be surprised if you find yourself grieving for him and what could have been too.  It is all part of the process.    Remember that in taking action to keep the girls safe, you are keeping other girls safe too.  I wish you peace.