Jean, this research is a little old, but it was a major study of genetics, twins and PD:

http://jama.jamanetwork.com/article....406#Joc81035f1

I think the main point for you is, if you don't have genetically caused PD (e.g LRRK2 mutation), there is no reason to think that your twin should be at any greater risk. I would moderate that opinion a little bit though and say, to the extent that twins share the same environmental lifestyle for an extended period of time, then there may be some increased risk as a result of environmental factors.