Look for and take advantage of "sibshops", special workshops designed for siblings of children with special needs (all types, not just autism). These are typically hosted by professionals or university-age siblings, with a strict "no parents" rule. They often are very open, fun events, with no particular agenda except to give the young people a forum in which to talk about their feelings and individual situations. Any sibshops we're aware of will be posted on our events page.
If you are running programming in the home, you may be able to involve siblings in some of the programming activities. For example, if you were practicing getting someone's attention, the sibling could easily be the person whose attention is being sought. They can also take part in games or other activities. Some children may find the tracking of data, or certain sensory exercises, particularly interesting.
Numerous books exist for sibling audiences. Here is one: "Everybody is Different: A Book for Young People Who Have Brothers or Sisters with Autism", by Fiona Bleach, 1991: The National Autistic Society, London, England. ISBN 1-931282-06-4. This is a short but excellent book, written in simple and direct language, and drawing lots of analogies to attempt to help siblings understand the worlds of the person with ASD.