Big sis thinks she has total control of mom but is only poa in case mom cannot. How can we get her to stop making decisions mom can make?

Family discussion. These are difficult family situations. Is your sister underestimating your mom's competence? Does your mom acquiesce too quickly? It would help for your sister to understand that eroding an elderly person's independence and decision making ability can speed that individual's decline. There are many good articles on line you and your siblings could read and discuss.