Signing a will while having dementia does not automatically make a will invalid. In order for a will to be valid, the person signing must have “testamentary capacity,” which means he or she must understand the implications of what is being signed. Generally, your mother would be considered mentally competent to sign a will if the following criteria are met: She understands the nature and extent of her property, which means she knows what she owns and how much of it; she remembers and understands who her relatives and descendants are and is able to articulate who should inherit her property; she understands what a will is and how it disposes of property; and she understands how all these things relate to each other and come together to form a plan. A qualified elder attorney should determine whether your mother has testamentary capacity to make any changes. If she does make changes, the will would not automatically be void. Someone challenging her capacity to change her will would have to file a will contest.