there are many alternatives all of which involve using either an elder law attorney, elder mediation to resolve family issues with the help of a skilled, trained elder mediator, or a combination of the two. Your mother may lack capacity to make a Will and an evaluation should be made by a geriatric doctor or assessment in an elder health unit of a hospital or facility to assess her capacity to determine whether the Wills are even valid. You can the file to be appointed her guardian.
It seems as if your mother may need to have a legal guardian appointed to care for her financial affairs if one had not already been appointed by the Court. You described that your sister had taken on this role, perhaps even without a court order, and that you have sued or are about sue your sister to have her account for mishandling your Mom's affairs over the last 4 years. You may bring a guardianship peition in Supreme Court and request that you be appointed as guardian. Once you are appointed, you will have authority to prevent you sister from forcing your mother to execute another will.