The National Autistic Society (UK) has some great advice for deciding when to explain to your child they are autistic and how to do it:

They also recommend some books to help you.  I like:

I Have Autism... What's That?

and this next book is useful for explaining to a sibling that their brother or sister has autism:

My Brother is Different: A Book for Young Children Who Have a Brother or Sister with Autism

Carol Gray at the Gray Centre has also produced a workbook for children with autism, so that they can learn about their individuality.  This workbook incorporates the use of social stories.