That depends ... having dementia alone is not sufficient to overturn a will. The real determining factor is whether your father-in-law was competent to write a will. Did he know he was changing his will? Does he know what property he owns and what he wants to do with it? Did he know the objects of his bounty (i.e. who is has a fondness for and would want to include)? This is what is important in Iowa. If you have concerns I would attempt to address them now. Talking with the person is a good start to determine if this is what he wants ... another option is to have a physician determine his level of competence. Perhaps it would benefit you to speak with a local attorney well versed in wills. Free advice will probably not answer your questions completely.