How would one prepare relatives for a holiday visit with a child with autism?

Too broad a question. Lots of unknowns in your question make it hard to answer: how old is the child? What is his/her age & level of functioning? Are relatives coming to your home or you to theirs? Do they already know about the diagnosis? How sensitive is the child to changes in routine & daily environment? Some need no preparation because they pay no heed to what is going on; others need relatives to "fit in.".