The stepdad, if he loves your sister, has to commit to being a stepparent to a kid with extra needs. That means he has to learn about the condition.
ASD kids will butt heads with people frequently if they feel like they are not understood - and they often spend most of their time being misunderstood.
No Cygnet is good for kids of any age...even adults! The book is also brilliant. Whatever the age, the system is the same.
The key really is understanding. If the child feels understood, he will become more compliant, willing and happy. If the parents understand, they will have different reactions.
We used to become very annoyed with my son over his insistence in doing things over and over after being told not to. It's exhausting.
When we learned how to talk to him, and when he got a diagnosis and proper support in school he changed completely.
He feels different. He feels misunderstood*.
For him, it might be that doing something you or I find very simple is absolutely exhausting, and he will often feel he is getting it wrong without understanding why.
Please help this little boy. His Mum might feel overwhelmed but there is so much help in this country if you look for it.