My partner & I have been together for 2.5 years now. When we started living together, myself & my daughter was introduced to 3 his teenage kids (twins - 15 years & girl - 17 years) mid 2020.
So in 2020 when the kids were 13 and 15, their father only introduced his children to you and your child AFTER you moved in together? Did you do the same with your daughter and only introduce her to your partner after they'd moved in. That choice always had the potential for disaster and two of them being autistic also adds more challenges, is there a reason he chose not to give the children time to meet you and your child beforehand? That's a lot for all the kids to have to navigate and 13/15 there'll have been puberty, lockdowns, and all the general covid shit too.
We all got on fine until I had a row with their dad on New Years 2022 & everything changed. His son took a shine to me before 2022 because he's autistic like my daughter but on different sides of the scale. I apologised about the row to them & he said "it was such a pathetic apology" to his dad. It upset me. So now, he lies about why he's not around us every 2 weekends & Wednesdays.
I've just seen the post about the content of your row, so his autistic son basically heard you complaining that he was there a few extra days, you can't see how that would massively hurt his feelings, especially as he "took a shine" to you?
I'm not surprised it's changed his relationship with you and he feels unwelcome. Just because you said sorry it doesn't mean he won't internalising those comments. You can't expect him to not feel how he feels after hearing that. You say he lies about why he isn't there every other weekend? What do you mean by that? Has he stopped having contact with his dad and what lie is he telling?
If you're rowing about the children I'd call it a day, is your own daughter hearing these rows too? I don't think the dad is putting his own childrens feelings first if I'm totally honest, it sounds more like he's digging his head in the sand and pretending not to see the emotional damage to everyone, you, your daughter and his children.
I don't think you're a witch. It sounds like the children haven't had the chance to know and understand each other and have all just been expected to muddle along and then the children are hearing hurtful comments from you both and it's not healthy. Putting the children, on both your parts, would be you both splitting up in my opinion.