I would take them to a workshop or information evening, or take them along to an appointment with the pediatrician

Try amaze.com.au they have so many helpful tips

It is really hard to get people to understand that Autism is NOT bad behaviour. Even doctors don’t know enough about it as yet but they do know that the pathways to the brain are wired different. Children with Autism don’t have a filter and see everything in black and whit not literally. They are also highly intelligent! See if you can get a letter off your specialist explaining what it is to show to the grandparents..

Ensure the workshops or sessions are aimed at grandparents; even go with them.

Maybe attend a workshop or introduce them to some books on Autism.

They are a generation behind,you need to sit down and educate them on the subject.Best of luck!

I am involved in diagnosing young children with Autism. Our tem is quite happy to offer a feedback session for extended family members to help them understand. This includes a description of the criteria and the characteristics of the individual child that meets the criteria. When it is personalized it will often help others to understand.