What is the probability of the sibling of someone with autism having an autistic child?

Remote: Autism is a spectrum disorder that is very complicated. Some genetic disorders may present as autism. There may be a carrier state. However, the details of genetic transmission are very poorly understood. If there is a specific genetic marker that can be found, then there is a better chance to answer your question. Without a marker it is not possible to give an answer.

Suggestions: If the child with autism had a genetic microarray & dna probe for fragile x, his/her parents may be willing to share the results. Also, a 3-generation family history of the prospective father and mother is warranted, not just for autism, but for other medical and psychiatric conditions that may be related. A prenatal genetic counselor (www.Nsgc.Org) may then be able to discuss risks.