Actually, you can't make them understand.
And since they have very little time with your child they will never fully understand.
Couple that with that most family don't want to hear your struggles or victories, it's a lost cause...
Unless 'THEY' live in your shoes.

There’s a saying.. “where there’s a will there’s a way”. If that person cares about you and the child they will. Or, there’s others like my oldest who just stays away, it’s easier for him. I’ve learned in time who’s there and who’s not..

Another wrinkle is when the person on the spectrum doesn't accept their diagnosis. When they are adults, and request that family not be told the details, it tends to put some distance and tension in family relations.

https://www.autismspeaks.org/

Autism Speaks

https://www.autismspeaks.org/

Educate. I think it's on Autism Speaks where I found information for grandparents, etc.