Hi, I feel for you and your dd. We have had similar situations with ds1 (age 8, ASD) and ds2 (age 6, NT). In fact we had a similar one today when ds2 accidentally knocked a gogo crazy bone off ds1's headboard and he completely lost it and scratched his face. Although in fairness, I have to say its unusual these days for him to do something that physical, he tends to be a lot more hurtful with words and also general screaming and shouting.
When ds2 was tiny, before we realised that ds1 had ASD, we thought it was just sibling jealousy. He is truly vile to his little brother sometimes and is heartbreaking to see as a parent.
The thing is, we know that they both absolutely adore each other really and these incidents leave them both very raw and emotionally fragile.
I hadn't really realised that it was down to rigid thinking until very recently. Obviously, I knew he was inflexible and a rigid thinker, but I never really considered that that was what was behind a lot of his outbursts.
We just had the EP in to observe him at school this week and she really helped me to get my head around it in her debrief phonecall. As she put it, he has a very narrow view of how things should be or how a situation should play out and if things deviate from that, he struggles to cope. He just can't handle things not being (in his mind) 'right'. Unfortunately, due to the lack of theory of mind, he is unable to understand that other people don't instinctively know exactly how he wants things to be, so that's why he explodes. She is going to be working on coping strategies with him over the next few weeks. She explained it as, because he has ASD, we can't necessarily change the rigid way he thinks, but what we can do is teach him new ways of coping with those thoughts, so his reactions to difficult situations aren't as distressing etc for both himself and other people.
The only thing that has worked for us so far, is a zero tolerance policy to physical violence of any sort. Any physical argy bargy and the instigator gets a time out immediately. (This is the bottom step of the stairs at home or the car with one of us stood outside it if we are out. If he can't calm down on the bottom step, he has to go and sit on his bed.) For ds2 (who is nt) this is a punishment, as he hates being separated from everyone else. For ds1 (the one who has ASD) its really just cool down time and invariably he comes back in as if nothing has happened and can't understand why everyone else is still cross or upset.
The other thing we have to do is try to explain how ds2 feels as if it were from ds1's point of view, by relating it to something he has felt or experienced himself. He is unable to think how his brother might be feeling when he 'goes off' at him, as he's just acting/reacting on impulse and totally in the moment.
So for example, in our case, ds1 has been on the receiving end of some bullying at school. We have had to use that to explain to him how upset and scared ds2 is when he verbally or physically attacks him. This really did help ds1 to understand how his behaviour affects his brother sometimes. Unfortunately, until he develops some better coping strategies, despite knowing this, he still can't stop himself doing it.
This approach does seem to work to some extent, although its still sometimes difficult to interrupt the behaviour when it 'erupts' and its too sudden to intervene successfully.
We try to do similar with good behaviour when appropriate and tell him what a lovely, kind big brother he is and how happy he has made his brother feel.