Requirements for a Valid Will
In every state, the most important prerequisite for creating a valid will is "testamentary capacity," a form of mental competence. Having testamentary capacity requires knowing how much and what type of property you own, knowing who your beneficiaries are and knowing how your assets will be divided among your beneficiaries. It also includes understanding that you are making a will, and what the effect of making a will is. State laws also impose formal requirements. In most states, for example, a will must be in writing and the signing of the will must be witnessed by two adult witnesses. Finally, the language of the will must express your intentions with reasonable clarity.
Terminal illness by itself doesn't prevent you from creating or modifying a valid will. The key question is whether or not your illness robbed you of the testamentary capacity necessary to create or modify a will. A purely physical ailment, for example, is less likely to affect your testamentary capacity than an illness that results in dementia.
Most state do not recognize an oral will as valid, and other states recognize one only if it is made as the final act before death -- by a terminally ill patient on her deathbed, for example. Typically, two witnesses are required. States that recognize oral wills allow them to dispose of personal property but not real estate, and place dollar limitations on the amount of property that they may dispose of.
Burden of Proof
When an apparently valid will is entered into evidence in a probate proceeding, it will be considered valid until proven otherwise. The person challenging the will bears the burden of proof. Since a probate proceeding is not a criminal proceeding, the challenger need not establish invalidity "beyond a reasonable doubt." Instead, the standard is "preponderance of the evidence," meaning that the challenger must produce enough evidence to convince the court that the will is more likely than not to be invalid. Even this standard of proof may be difficult to meet if witnesses testify that you were of sound mind when you signed the will.