There are a number of legitimate reasons that one or several people may want to challenge a will, and each comes with its own set of hurdles to clear. In general, challenging a will is not a simple matter and is not to be undertaken casually. Almost all wills end up passing through probate just fine, with only about 1 percent being challenged. However, if you believe that a certain person's will is invalid, there are grounds on which it can be challenged. One of the most common grounds for challenging a will is bringing into question the testator's ability to have written the will.
Challenging testamentary capacity is usually sought by claiming that at the time of the will's creation, the testator was somehow not fully in control of his or her mind for the purposes of making sound judgement. This can vary from claims of mental illness or dementia to being under the influence of a substance or some other form of duress that would have compromised the testator's ability to make decisions.
In order to successfully claim incapacity on the part of the testator, you will need to be able to prove that the individual in question was not able to understand several key elements of will creation. These include understanding what a will means and how the statements in the will may affect those individuals the testator may bear responsibility towards, as well as the value of the estate and how all of these things work together in relation to the distribution of assets.
If you are able to prove that the testator was indeed not able to make sound judgement in this case or was not able to understand the nature of his or her decisions, then a challenge may succeed. These matters can be very delicate and require handling with care. It is usually wise to approach such a matter with the guidance of an experienced legal professional who can help you understand the scope of the matter.
Source: Findlaw, "Reasons to Challenge a Will," accessed Dec. 08, 2016