I'm back, excuse the flouncing it was all getting a bit much this morning.
Social stories are a good idea. I haven't begun using them yet but will. This was also a suggestion by DS' education psychologist who works with us in conjunction with nursery (to use in general, not specifically for the violent outbursts)
At present, I use a 'now and next' board to help with transitions which are a meltdown trigger. This was a suggestion given to me by a SALT therapist and works well on a day to day basis. I have 20+ cards laminated (including one with a picture of his sister on) which I put on the board, with him, to let him know what's happening on any given day.
With regards to what OH is doing to remedy the situation, the answer is nothing short of separating them and dealing with the behaviour there and then when it happens. It's me who is fretting about the situation in general and wanting to find ways to prevent, rather than just handling it when it happens. He's much more laid back than I am and doesn't tend to 'worry' like I do.
I appreciate the input from those of you who understand autism and/or have autistic loved ones too. As you will appreciate his ASD adds another layer of complexity as he just doesn't respond to being told off in the way a NT child would.
He's semi verbal at present and doesn't have a good level of understanding and communication. He can tell me what he needs with one or two short words but there's no back and forth. If I give him an instruction he's not able to process that or follow it.
I just want to reiterate that my OP comes from a place of concern and care, despite the circumstances of her conception she's an innocent child and doesn't deserve to be attacked every week.
To answer the PPs who asked, she does have her own toys here yes. DS has a select few toys that he's obsessional about and doesn't want to play with anything else, fortunately she has no interest in those toys so there isn't conflict over sharing.
I maintain that I don't harbour any animosity towards her and I'm very fond of her, although perhaps I haven't been acknowledging just how difficult this situation is for me and DS.
Ever since she entered our lives I've been in 'Mary poppins' mode, so to speak.
Making sure the children are as happy as possible, making an exerted effort to welcome her and help her feel secure in her second home.
During the early days when I first met her I did have to excuse myself for a few minutes to have a cry but quickly buried those feelings and just got on with it.
Over time I became very fond of her and grew to look forward to her visits, she grew very fond of me too and preferred my company to OH's much of the time.
I can see why my choice of wording might have come across as harsh but I was using those words as a description, to be factual, not to be nasty to her.