If there is an autistic child or sibling or other family member, is there reason for gene testing the family?

Possibly. If there is a known genetic abnormality in the person with autism, then family members of child-bearing age may benefit. Even if physical & neurological exam reveal no signs of a specific syndrome in a child with autism &/or intellectual disability, genetic microarray & dna probe for fragile x reveal an abnormality about 15% of the time. Evaluation by a geneticist is needed in those cases.
No. Genetic testing would not provide any additional benefit in predicting who else would be at higher risk for autism.
Test the child first. Before testing family members, have the child tested. there are many disorders that can present with autistic features. If an inheritable abnormality is found, then a discussion with a geneticist should be done to determine who might benefit from genetic testing.