Dear bitbap, you have every reason to be looking for support on these pages, and I'm sure you will find it.
I have three relatives who all have ASD/ASC and the big comfort, now that the youngest are young adults, is that they have each other for the small parts of their lives that the rest of us don't exactly 'get'. It isn't like their interests are similar, but they are never going to be alone with no-one else who understands. So in a decade's time that may apply to your dd and dh.
But getting back to you, and now, it is important that you look after yourself, and that you don't feel bad about taking time to adjust and to grieve. It will get better. I found counselling and anti depressants handy, and a bunch of other stuff I put on a thread I'll post in a minute.
Your description of dd and dh setting each other off sounds very like my dd and dh, who can shout at each other for much of every morning before school. They are somehow too similar - neither are organised, both have a view about how best to be organised, and both explode. But they still love each other, very much.
In most households it is the person with ASD/ASC who is the odd one out, but for you it is the other way round. But as things settle down, families move towards a happy life accepting everyone as they are. It takes time, but after an initial period of unease, I have now got totally used to the way all the wider family does things.
I really believe that I have as much in common with each of my ASD/C relatives as I have with my nt relatives. From the big stuff like 'We all care about each other', and 'we all feel passionate about something', and on down right through to the little things like 'we all like chocolate cake'.
And actually now I think about it we all have as much that separates us: my nt sister is a workaholic and a perfectionist (not been accused of that) my nt and wonderful mil is incredibly arty and can't send an email, my ASD relatives communicate slightly differently from the mainstream, and I happen to use a wheelchair. And it is so odd to find myself writing this because none of us ever really think about what separates us - my instinct is: "so what".
You are in an uncomfortable patch of change, of moving your expectations to adapt to a new normality, a different one from the one you imagined, but it will happen, really, and you will get there.
It might be that the new normality still involves dh and dd still shouting at each other every morning, but you will all be totally used to it still love each other, and that is what really matters.
I hope this helps, and if it doesn't than you can quickly put it out of your mind.