A scary thought Ann. My youngest Grandson is autistic.

I did what research I could on any relationship and only found these references, but nothing specific to grandparents.

I found this abstract, but it does not comment on a connection:

Résumé / Abstract
The pathogenesis of Parkinson's disease, a neurodegenerative disorder, is multifaceted, having a variety of genetic and environmental factors. There is considerable evidence to support the role of toxins, particularly pesticides and herbicides, in at least some of those affected (presumably, mostly the genetically vulnerable). The pathogenesis of autism is no less complex, but little is known about the potential role of toxins for autism, a neurodevelopmental disorder. The incidence of autism appears to be rising, and early exposure to synthetic chemicals is one suspect for this rise. Impaired detoxification of certain chemicals may be common to autism and Parkinson's disease. Further study of environmental influences for either disorder may lead to important insights regarding causation for both, and perhaps for other neurodegenerative and neurodevelopmental disorders as well.

I did however find information about Vitamin D deficiency in Autism, which is also found to be a deficiency in PD.

Autism and Vitamin D
Researcher sees link between vitamin D, autism
Abstract: Autism and vitamin D