Your sister has it backwards. Guardianship trumps POA. POA's can be removed by the person but guardianship can only be removed by the court.
However, your mom still has some control over her life with a POA and can make decisions if she is competent. However, if there is guardianship I am assuming your mom isn't competent.
The only way your guardianship will mean nothing is if your sister takes you to court to fight it and is granted guardianship herself.
My understanding is that if there is in-fighting in the family over guardianship, the judge will generally appoint an outsider Guardian. What is the argument between you and your sister? Do you differ on what your mom's needs are right now?
However, your mom still has some control over her life with a POA and can make decisions if she is competent. However, if there is guardianship I am assuming your mom isn't competent.
The only way your guardianship will mean nothing is if your sister takes you to court to fight it and is granted guardianship herself.