Unfortunately, it's not easy to find out if someone holds a power of attorney given by another person. I know of no law that requires that a person who holds
the POA (called the "attorney-in-fact") must show the document to concerned people, including (other) family members.
However, there are a number of actions you could take. First, you could ask your mother, assuming she is mentally competent, if she has given a POA to her daughter, your sister. If you mother says she has, you could ask her if you could look at it. You could also ask your sister to show you the document, but I gather that she would be unlikely to do so. Finally, if you suspect that you sister is abusing her authority—either by not actually having a POA, or having one but not using her authority in the best interests of your mother—you could tell your sister that you want to read the POA and, if she refuses, you might have to file a lawsuit against her for abusing her powers. Lawsuits are alway a nasty and expensive business, so I suggest that you file one only if you are quite sure that your sister is doing things which cause harm to your mother.