What should I do or is there anything I can do after my sister spent my mom’s SSI benefits?

How is it possible for your sister to spend benefits? Did your mother have a debit card? Did your sister know the PIN number and use it? Or did your sister write checks and sign your mother's name? Your sister has engaged in criminal conduct. Since nobody has a POA, if your mother is not mentally competent someone has to start guardianship proceedings immediately. Someone should notify the bank to see if they will freeze your mother's account or prevent your sister from accessing it. If necessary get the debit card shut down and/or get things moved to a new checking account. If a new account is established, SS will have to be notified to change the routing number. The problem is that if your mother is not mentally competent then the bank may not be willing to act on your say so alone. In that case you may have to get a temporary emergency guardianship order from the court. Your mother or her guardian will then have to contact the district attorney and have your sister prosecuted unless the funds are restored ASAP. Once criminal charges are initiated, your sister can only pay back the money as part of criminal restitution or some kind of plea deal. So she either does it now voluntarily or she has to deal with the criminal justice system and a conviction.