It sounds as if you and the other family members have given up in exasperation due to your sister’s actions and inactions—and that is understandable.
However, it may be worthwhile
to try to have one family meeting in which you try to reason with your sister, explain that you are all concerned with providing the best care for your mom—and ask whether she needs help in carrying out the serious duties of acting as your mother’s financial agent.
If that approach doesn’t work, and your sister persists in her bad behavior, then she may actually be committing financial elder abuse—and you should take action to stop it at once. Your first stop should be to contact the local Area on Aging.
Representatives there should provide a confidential assessment of your situation and refer you to additional local resources for help if that becomes necessary.
If it becomes necessary to remove your sister as your mother’s agent under the power of attorney for finances, be sure you have someone in mind who is ready and able to take over the job.