Do you have any suggestions about the best ways to try to explain autism to an intelligent 6yo who assumes an older child’s anti-social and arrogant-appearing actions are intentional?

It depends on whether there have been boundaries taught to the older child with autism. What I mean is, if the older chid says something inappropriate, are the adults able to point it out to the older child followed by correction (apology, begin appropriate behavior) or redirection to another activity?

This isn't a judgment on a parent: it can take years of compassionate teaching to develop socially appropriate behaviors, and significant progress may not be possible for some.

But it’s going to be extremely difficult for a six year old’s sense of fairness if the older child “gets away with” behaviors that would receive immediate punishment if committed by the six year old.

On the other hand, empathy and compassion for others is an invaluable thing for the six year old to learn.

Fair isn’t equal.

I think I would first talk to the older child’s parents to see how much information they are comfortable being shared about his autism with the six year old.

There are plenty of kids books that talk about famous people with autism, such as the inventor of Pokémon, as well as kids books on the sensory and other aspects of autism. So an intelligent six year old could certainly gain a better understanding of the why behind the older child’s behavior.

My thoughts, FWIW.