It is certainly possible for more than one child in a family to have aspergers just as it is possible for more than one child in a family to be blond or be left handed.
Research is showing that aspergers develops in the womb where there is too much testosterone. I don't know where this extra testosterone comes from.
Genetics also play a big role in aspergers. If your child has it, look to your husband, father or father-in-law or even a generation back. Chances are you'll find a male who had undiagnosed aspergers.
While children do learn from their older siblings, they also learn from their friends and family. I've never had any trouble with my daughter picking up asperger traits from her older brother.