Autism is almost certainly polygenic, that is to say there are lots of genes responsible for the final expression of the ASD state. Although ther have been several gene sequences shown to be associated with certain ASD traits, they are far from diagnostic. While there may be some value in testing to rule out other “autism like” conditions it is limited and the predictive accuracy for sibings is poor.
While evidence is mostly anecdotal, mothers are noted to have at least some traits but that does not rule out paternal involvement (interesting as it is usually reported that incidence in males is higher). If we consider that well over 25% of the general population show autistic traits at some level or other, attempting to assign the genetic source of the condition is probably futile. I personally believe that the condition is semi-randomly created when the sperm/egg is produced or shortly after fertilisation and not simple mutations passed from parents. This would be very difficult to test for using current technology.
To be honest, unless you wish the results of the test to be used to further understanding of the condition, I’d say it is pointless, a considerable waste of time/money, and certainly not helpful.