This is the million dollar question. Unfortunately, there is no million dollar answer. Everyone's PD manifests itself differently and the drugs that we take affect each one differently. And it all can change seemingly for no reason at all. But, advanced age plus length of disease may increase the risk for dementia.

I hear the frustration of your simple question. Believe me, after 18 years of this disease, I ask it myself along with its corollary: Why would "it" just stop?. But, my better angel reminds me that we have come as far as we have in our understanding and knowledge of PD because we continue to ask these seemingly simple questions in the face of our personal struggles. Thank you and I wish the best for your family.

(And my better editorial angel makes me realize that I turned this answer to your post into my own soliloquy on my own simple questions. I am sorry. This is not a simple thing that you are dealing with, not at all. I do not have an answer for you except to say that it sounds like your FIL is a fortunate man to have such a caring family.)