Yes, you can dispute the change and challenge her competency. But, that gets pretty complicated. Expert testimony may be needed. You need to contact an experienced attorney to help you sort through this. Best wishes.
Attorney Odle has provided you with solid advice. It is very important that you meet with an experienced probate lawyer so he or she can begin to document your mother's mental illness prior to her death. The attorney may also advise you as to whether your mother should have a fiduciary appointed for her at this time by a New York Surrogate's Court. Finally, once your mother passes, the attorney will be at-your-side immediately to defend your interests in your mother's estate. In short, you need a lawyer as soon as possible. Good luck to you.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.
While your mother is free to leave her estate to whomever she pleases---she has no obligation to leave anything to her children or any of them, if she has recently changed her will and you believe there was "undue influence" or "lack of mental capacity" you should have a lawyer review the situation as soon as possible.
My colleagues have advised you well as to the issues. We all advise getting a lawyer asap. Will contest can be very messy, and often take years to settle. Try to address this situation before your mother passes away.
This answer is provided for informational purposes only and it is not intended as legal advice. Additionally, this answer does not create an attorney-client relationship. If you wish to obtain legal advice specific to your case, please consult with a local attorney