Undue Influence
In addition to mental incompetence, undue influence is a way of invalidating a will. Often, undue influence goes hand-in-hand with mental incompetence. If one heir is proven to have taken advantage of his or her benefactor's senility or dementia to get changes made in the will, there is undue influence. This can be hard to prove. You may wish to consult a legal adviser about the standards for contesting a will where you live.
