Please dont leave him alone to deal with those emotions 😢
Why can't one of you remove DD and one of you stay with him until it passes, he will be so overwhelmed he should have someone there. All of you leaving sends a terrible message and I'm afraid it won't help the dynamics at all he will just come to resent you all especially his sibling. If you find you are in a situation of having 1 parent to two children and it kicks off then parent needs to make sure that DD knows when to back down and walk away instead of focusing on physically protecting her, stay with him, focus on him help him calm down he needs to be redirected, lead him into another part of the home, somewhere not too stimulating and somewhere he feels safe and wait for it to pass even if you have to sit in a room with
your back against the door, take him there everytime while your DD quietly removes herself, she has to take herself to sit somewhere and not get involved. He should soon learn to go himself when he feels the need to.
As for the physical stuff you really need to reinforce it while he's calm...there's no point trying to reason with him once he's already in that state. You need to help him identify what he's feeling before he attacks, make a plan to redirect let him have an input on this and agree some consequences will be put in place for hitting others and make sure other DC have some rules/consequences too so he's not singled out.
I would also avoid trying to communicate too much when he's having these episodes, keep instructions simple, do not get emotional and don't try to apply any punishments at that time just focus on helping him to get over it.
I'm not one for sending them to punch something else because I don't want my DS's thinking punching something will make them feel better.
We use sensory aides instead...theres loads out there, aggressive trampoline jumping or pushing against a wall/floor works a charm with one of mine, the other has a gym ball that he fling himself on and also likes to squeeze things. Basically our own ways to apply deep pressure techniques because they gave the same feeling that the child was seeking by attempting to hit.
It might be worth doing a sensory checklist to see if there's anything there that could help ?
DS1 that used to attack my DD actually benefitted from a bear hug (helped with the flying fists and gave me a few seconds to sort of push him in the direction I needed him to go) Sometimes he would just give in to the hug and sob, other times he would struggle but it gave DD time to get somewhere else, like completely out of his sight. DS1 also got given a giant heavy duvet to wrestle with while he screamed it out. I would say ages 9-13y were the most difficult, he is now 15 and regulates himself a bit better by first removing himself then either does laps up and down the garden or if he can't get out there' he basically has a tantrum in his room then comes back to us later, sometimes apologises, we'll have a drink then we'll chat about how we could have hanlded it better and praise what he handled well...it's not always perfect but he doesn't lash out at anyone physically anymore. Even outside of the home if anyone starts on him he simply walks away.
My other DS is younger and non verbal but I have learned to identify when I need to remove him from a situation...and I am still learning how best to support him.
It's not too late for you to learn what your son is trying to communicate to you.
Not to place blame but is DD saying or doing anything that is causing him to be so upset. I saw you mentioned he is a sweet and affectionate boy usually so I would wonder what the dynamic is between the two. I would try to get to the root of that first to be honest. In our case there was jealousy and feelings of inadequacy bubbling away so when DD was on a wind up DS1 just popped instantly!
We didn't realise at the time she had been saying snarky comments and teasing him when we were not in earshot, telling people his personal issues and getting her friends to tease him at school.
She was told that was not acceptable and received consequences for it.
They have both had to learn what "banter" is vs bullying.
If something like this happens and he has an aggressive reaction they will both receive a consequence and a lecture.
The focus must be on identifying when your DS is feeling overwhelmed and redirecting until he is able to understand and actually remove himself.
Anytime he is able to do this even if just for a moment be sure to praise him for it later.
Now is the time to work on these behaviours before he's a know it all teen because that will be a struggle!
I wish you good luck and lots of patience x