If the POAs that you are referencing are medical POAs , they should be helpful in the situation that you set forth here. One thing that you should know is that only a person with POA over sister can act on sister's behalf. If your mother is unable to act for sister under sister's POA, (due to her being sick now) the alternate/contingent attorney-in-fact in sister's POA would be able to act for sister. Your brother being POA over your mother has absolutely nothing to do with your mother being POA over sister. They are 2 different people and 2 different POAs.