I guess I should added My youngest is the one on the spectrum. My older children are five and eleven year older than she is. So they knew something was going on before the diagnosis of any of the disorders she has. So it was easier for me to talk with them. And to add my 16 year old is in counseling now to help deal with his sisters behaviors.

I have two ASD children and my daughter the younger one is severly autistic and her older brother who is high functioning asks why his sister cannot talk and why she does what she does, and we started telling him that his sister is younger so she cannot do the same things yet. Once that started not to work anymore we started telling him that she needs more help in school which he took pretty well. She is now in residential so when he started asking about that we told him it is like when kids go to college and she has to have more school time than him, and he has taken this well to.

We have not told him he is also autistic because we do not think he is ready, and he is eight. I guess what I am trying to say is you have to do things slow and only tell them what they are ready for. You know your child best so do what you think is best.

There is a kids book out there. I think its called A WALK with IAN. I just explained to my kids that their sister thinks differently than they do.

It is a complex subject and it can be difficult to explain. My four year old has a tough time understanding and that makes things tough for us as parents when they fight because it is almost always because little brother who is typical functioning wants desperately to interact with his 6 year old brother. I have heard of some organizations that do things called 'sibshops' which are workshops where typical functioning siblings can go to learn and understand about autism. We are in AZ so I am not sure what is available where you are. I would also suggestion some books (these are better if your child is a bit younger)
1. My Brother Is Different: A Parents' Guide to Help Children Cope with an Autistic Sibling by Barbara Morvay

2. All About My Brother by Sarah Peralta

3. Everybody Is Different: A Book for Young People Who Have Brothers or Sisters With Autism by Fiona Bleach

4. Siblings of Children With Autism: A Guide for Families (Topics in Autism) by Sandra L. Harris and Beth A. Glasberg

5. Sometimes My Brother: Helping Kids Understand Autism Through a Sibling's Eyes by Angie Healy

Pray this helps.