How can I relinquish a medical power of attorney with no one else to take over?

As both your sister and you are in Pennsylvania you should consider quickly obtaining a consult with a elder law attorney there for your options or an elder care attorney to represent your sister. There is probably the equivalent of adult protective services, someone who can step in for a vulnerable adult, and they may need to be contacted. Hope you can get it worked out.

I do not understand how you can be a power of attorney by some agency for some other person. What did you think you were agreeing to do when you agreed to be poa? An agent under a health care power of attorney is responsible to make health care decisions for the principal (your sister) if she is no longer capable of making decisions for herself - she may have some condition which precludes this like dementia or Alzheimer's. she does not have to be comatose. if you did not understand what you were doing you should have talked to an attorney before you agreed to be the agent. However, renunciation is simple - you have to send a certified letter to her (i would also include the nursing home and send this letter certified as well) advising that you were named as agent under the poa and that you no longer wish to be the agent and that you are resigning your duties immediately, effective upon receipt. the county will then have to appoint a guardian over your sister. This is very very sad. If a public guardian is appointed, they are not necessarily going to make decisions for the benefit of your sister or have these decisions be in accord with her wishes. but if that is what you want, then no one can force you to be an agent under a power of attorney.