how do i stop my sister from getting power of attorney over my mother? my mother has dementia....

Not sure exactly what your question is.  If your mother is incompetent, any power of attorney she grants your sister (or anyone else for that matter) is invalid.  You can bring an action in the appropriate court to have the power of attorney declared null and void or, if it has not been granted yet, to enjoin such a power of attorney from being created.  You would have to prove that your mother is incompetent.  However, if what you are asking is how you can prevent a court from declaring your sister the guardian or conservator of your incompetent mother, and presumably that you be appointed instead, that is a much different and more complex question, depending on such factors as your sister's and your trustworthiness, financial and business experience, familiarity with your mother's financial affairs, etc.  If your sister has applied to be appointed as guardian or conservator, and you agree that your mother is incompetent but want someone other than your sister to be appointed, the Court will have to choose which one of you, or someone else, should be appointed based on factors like the ones I've listed.