Hi OP - I'm sorry you are experiencing this awul problem. I've hesitated to answer your post because I wasn't sure if it would be helpful or move the discussion forward in anyway for me to do so.
I was the little sister in your scenario. My middle sister exactly like your son was the initial target, then I was born and it was all transferred to me. i grew up in absioute terror of my oldest sister and it has afected me all my life (low self esteem, self harm, anorexia,marrying a very abusive partner because I was so used to abuse) my middle sister spent her childhood trying to protect me and is extremely protective of me even now (we are 59 and 63 - you can see how long the effects have lasted)
I can remember my mother in tears screaming 'I can't cope with this' as my sister attacked me yet again. I had everything - pushed dowstairs, locked in cupboards for hours, woken in the night with a knife to my throat, pillows held over my face, boiling water thrown at me, hit every single day, verbally abused every day etc etc etc. Anything and everything would set her off, often for something like 'I just hate your face' or ' I don't like the way you smile' 'your laugh is stupid. never laugh again or '
I was told ' all sibling quarrel'. But they don;t not like this. This is not normal and anyone who thinks it is has never experienced this level of abuse from a sibling. I think you are very brave in recognising that this is not right, your DS's behaviour is way outside the boundary of normal. I applaud you for seeking professional help - I hope very much you get some help and recognition of this for the serious problem it is.
My parents tried everything, - harsh punishments, huge amounts of attention, rewards for good behaviour, anything and everything they could think of. Nothing worked.
As ana dult my sister has been diagnosed with a personality disorder, it is something beyond amentla illness that she has. I am no contact with her now, I went very low contact many years ago so she could never have anything to do with my children and harm them. On the few occasions I have seen her (family funerals etc)her behaviour has been beyond bizarre, she is a compulsice liar and still very abusive and aggressive in her behaviour and her outlandish reactions to anything.
I think the point I am trying to make with this ramble is don't give up on trying to find out what the problem is with your DS, it isn't normal, don't let anyone fob you off with 'sibling rivalry'. You aren't a bad parent - you have an extremely difficult child. No doubt you could (as we all could) try out different parenting approaches, no harm in trying, but I wouldn't set your hopes on them working (and I'm usually the first to offer techniques that work, - I've worked with children with behavioural problems).
Push as hard as you can for referral, get SS involved if you think that would help you, put your DDs first (and by putting them first and getting your DS help, you will help him as well).
I'm really sorry for you, it is a horrible situation for you, don't let some of the responses on here make you feel under attack. Most people are trying to help, but I believe this is beyond simple tinkering with parenting techniques, your son has a problem that needs expert help.