My youngest started asking questions last year and we were just straight forward with him. They can handle a lot and that became apparent one day in the car when my oldest and I had this conversation.
Oldest son: "Mom, I have aspergers right?
Me: "yes"
Oldest son: "And Lance has PDHD"
Me: "that's PDD-NOS and yes"
Oldest son: "then what does Landon have?"
youngest son: "I'm the weirdo in the family because I only have allergies"

My son does not have siblings, but close cousins. They are 8 and 10 so I explain things as they are... in the most possitive manner possible. Being open about it with them was great, they feel now that they can ask more freely. They continue asking as they get older as they are in their way to understanding the world.

We're going through this. Daughter 1 (ASD) and Daughter 2...one year younger. They will attend the same school and daughter 2 has begun asking questions. We try to be straight forward. She's expressing concern about what her friends will say, questions they may ask. We (me + husband + daughter 2) are meeting with Daughter 1's psychologist today to answer any questions.

I got a bunch of books from the library for my two older children when their little sister got her HFA diagnosis. The one they liked best was called "All Cats Have Asperger's Syndrome".

I have four kids and my youngest was diagnosed at 4. I told my kids the diagnosis (they were 6, 7 and 9) and together we learned about it. It's almost a relief for them to know why their sibling isn't "normal"