Having dementia is a medical diagnosis that can provide evidence for or support a legal determination of incomptency--but it is not itself a legal determination. To invalidate a new or changed will, you would have to bring a legal action challenging it, in which you will provide evidence (e.g. medical testimony from her doctors) that she was mentally incomptent at the time she made the will. Only a court can determine that someone was not competent--either retrocatively, when they made the will, and so invalidate it; or presently, if the person is still alive, and appoint a legal guardian for her. This is not an easy showing to do: you'd be advised to retain a probate attorney to help you. (The lawyer can also advise if, given the expected size of the estate vs. the likely cost of the lawsuit, including the medical testimony--you'd have to pay the doctors for their time--it is worth doing this.)Note that a will only takes effect on death and can be changed at any time up until death: therefore, you can only challenge it after death, when it is offered for probate. If you feel that your mother is not getting the proper care, etc. from your siblings or that they are not acting in her interest and that she is mentally incompetent, you might want to consider bringing an action now to have her declared incompetent and have a guardian appointed for her; if you wish to explore this option, consult with an elder law attorney.