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. It may be prudent to obtain a note from her treating physician attesting to her capacity to understand and sign the will.
For more information about capacity requirements for executing various estate planning documents- click here.