Advice on how to deal with sibling being a trigger

Hi everyone

I am a newbie so forgive me if this topic has already been covered.  My son who is 5 has this week been diagnosed with Asperger's.  From what I can see, the trigger of his 'behaviour' seems to be his sister who is 3.  They are an absolute nightmare together.  He keeps lashing out at her, pushing her and bossing her around to the extent that he has a meltdown if she won't do what he is asking her to or play the way he says.  My husband and I quite often have to take them to do different things just to reduce the stress levels. He is generally quite good when it is just him or him and his little brother or when there is another friend around, but if its just the 2 of them or all 3 siblings together then all hell breaks loose!  Does anyone have any advice on how to deal with this?  Due to it being a recent diagnosis we haven't yet got to grips with his sensory triggers or how to deal with 'discipline' for his actions and how to make the siblings understand when they see him 'getting away with things' so any advice/experiences would be appreciated.

Thank you x