A mom on the Deadwood moms' email group recently requested advice on how she should get ready for a visit from a relative and that relative's tweenish autistic son. Since I am not one to shut ye olde yap, here is my response:
You are very thoughtful to even send out this query. However kids with autism are all very different from each other, so what works for one kid will not necessarily work for another.
The best advice I can give you is to try to create a safe space for your relative's son to retreat to if he needs to. Even if it's just a chair, though a room (even if he is sharing it) would be ideal, so that his mom can retreat with him. Also try to help your kids understand that it is probably best to not surprise their cousin in any way if they can help it. Your littlest may need your help with this goal. (Though, again, it might not be an issue.)
But, truly, I would ask your relative directly for as much guidance as she can give you. If she is like most autism parents I know, she will be grateful that you asked. Be frank, and be open to her suggestions. Is he verbal? If so, what kinds of verbal interaction can he tolerate? Is he conversational? What kinds of social situations does he like/dislike? What kind of activities does he enjoy/detest? Does he have any "triggers" that set him off? Is he aggressive? Does he have any behaviors that you should tell your kids about, so that they'll know that those behaviors are totally normal for him? Are there any special foods he likes? Does she have a routine that he can implement, even partially, at your house?
I have some books for young kids about autism and hanging out with autistic kids, and would be happy to let you borrow them. I will be back from Kauai in two days (we have discovered that our son will tolerate travel if it is to a swimming destination), so if you like I can contact you then.
Good luck, and have fun. Most kids with autism have an immense capacity for pure joy.