Because of my son being born with a diagnosis within the Autistic spectrum I have personal experience with the study below. My personal opinion as to why a second child is quite often diagnosed in the same family is that with the first child you have no clue, but by the time the second child comes along the child does not remain undiagnosed as long as your first because you now know the symptoms, the things to look for so medical intervention occurs at a earlier age.
The following study was downloaded from the daily beast
Siblings of children with autism have a higher risk of being diagnosed with the disorder than once thought, according to a new study. The research, funded by Autism Speaks, analyzed more than 600 3-year-olds with an older autistic sibling and found that almost one in five of them eventually developed autism spectrum disorder, which includes Asperger syndrome among others. The findings are cause for pediatricians to keep a close eye on those siblings and use early intervention tools such as therapy to keep symptoms at bay. One in every 110 U.S. children has the disorder, and that number is unlikely to decrease anytime soon. The research is likely to call for more work to figure out what kinds of interventions are most helpful to children who begin to show signs of autism
The following link is from the blog AUTISM SPEAKS ,it is a a amazing follow up live chat with experts in the field http://blog.autismspeaks.org/2011/08/16/increased-risk-live-chat/