@donutqueen11
Husband is sons biological dad. He absolutely adores school and is appreciated for who he is so no issues at school at all. Academically he is flying and he has a fab group of friends - the teachers love him - he thrives on routine and is a credit to the school. He is so polite and well mannered - it is just at home he is so rude and verbally abusive to H and anyone who sort of mocks or doesn't understand his quirks or obsessions.
Does anyone include DD, i.e. is her brother verbally abusive to her too?
When you're not an only child, you have to share importance with your parents. When you're a neurotypical child and your sibling is not, it can feel like you're never at the top of the priorities list, and you're never the more important one. Although leeway must be made, with siblings, they should always be more important from time to time, e.g. assuming they don't share the same birthday, that's at least one day a year when one should get the spotlight more.
What I'm trying to say is that maybe a break away from her brother and some one-to-one parent time would be good for your DD. The one-to-one parent time might be good for your DS too (although I'm aware that the 'harder' parenting has been dumped on you without consultation, and that's really not fair).
Separately, there is the issue of what to do about your DH, because as the grown up in this scenario, he's not allowed to have enough and run away. I think it's positive he's taken your DD, as it means he hasn't abandoned all parental responsibilities, and if he thinks he's protecting her as well as himself, he's not an unfeeling brick.
Yes, the whole thing is a mess, but if he's still acting as a parent to one of your children, there might be a way back from this. He might just need some extra support from a third party.
Would it be possible for your aunt to spend some time with DS, and your BIL to spend some time with DD, so you can your DH can meet up on neutral ground and have a chat about what you both expect will happen next? Does your DH really think this is the solution, or is he just taking DD away to give her a break? How does he think they can reintegrate into the family home after making your (and DS?) feel so unloved?
I think some family counselling might be good, if he's willing to try that. And whilst you're doing a marvellous job in parenting a child with ASD, he's clearly struggling - are there any support groups/resources he can access? It's OK for him to find it harder than you. It's just not OK for him to give up and leave it all to you. He might have a personality clash with his DS, but he's still his dad. Running away doesn't change that.
I know you're tempted to now run away yourself, but as you said, your DS's school are great with him. I wouldn't want to lose that stability and structure, especially when the rest of your family life is so uncertain right now.
Be kind to yourself too. This is all a lot to take in, and I'm not surprised you feel overwhelmed and upset - it sounds like you've spent well over a decade being 'the glue' and that is such a hard job. Even 'the glue' melts sometime. If you're working, take some time off so when your DS is at school, you have have a little cry and and process your emotions. You need some time for you too. The glue is important.