Unlike the laws in many other states, Texas statutes do not spell out formalities for removing an agent in a power of attorney. As a warning to the wise, be
wary of shelling out money for the services offering to sell revocation forms.
The best and legally safest route is to ask your mother to create a new power of attorney, as this unequivocally revokes all previous ones. The new document should be finalized with all the same formalities as the original. For example, in Texas, powers of attorney authorizing agents to deal with real estate must be recorded in the office of the county clerk in which the property is located.
However, if your mother is unwilling to make this change or is unable to do so because she lacks the required mental competence, then the reality is that you cannot be forced to act against your will.
If it comes time for the power of attorney to take effect, your sister will simply take on the task alone. If you have been appointed the primary agent or the document specifies that you and your sister are to act jointly, some institutions may require that you specifically state that you are "unable to unwilling" to act as agent, just to be sure she is not trying to usurp your authority.