If I were you, I would definitely make an appointment with my doctor as soon as possible. Your concerns are valid and the only way you are going to know for sure is if you speak with your doctor and find out exactly what your chances are of developing either of the diseases based upon your family history.
My own grandparents also suffered with dementia. I'm now wondering if I should schedule an MRI, too. I am 47 years old and would really like to know if I am already showing signs of white matter hypersensitivity. If so, I would amp up my savings, insurance coverages and 401K plan contributions to make sure that there is enough money to pay for my medical bills and my family's well-being if I do indeed develop dementia.