My sister has medical POA and wants me to have it. How do we go about transferring it?

A Medical POA cannot simply "pass-it-on".
Who does your sister have MPOA for?
MPOA is assigned to someone by a "principal".
That is to say, if Sister is MPOA for her Mother, then it was mother who conferred MPOA upon her, asking her to make medical decisions when/if mother can no longer do it.

If Sister no longer wants to make these decisions then she can resign.
If mother is still competent then mother will appoint you to be MPOA and the papers are filled in with your name.
If mother is no longer competent in her decisions she cannot appoint a new MPOA.
If that is the case, are you listed as a "second on the document". There usually is someone who is listed if the first POA cannot serve.
In absence of that, the GENERAL POA would act.
In absence of THAT, then the next of kin would be consulted as to how the doctors should/would proceed if mother were to be unable to make her own decisions.
Simply SHOWING this document to them would express the wishes of the mother via an advance directive in most cases of an aged adult.

Remember, the more information you can give us the better we can help you.
In this case it would be:
1. Who is your sister POA for?
2. Is that person still competent in her own decisions?
3. Who is listed on MPOA as a "second"
4. Who is the general POA
and etc.
Wishing all of you well.

Why does sister want to revoke her Medical POA? Of the two POAs, Financial being the other, it has the less responsibility. The responsibility of the Medical is to carry out the wishes of the principle if they are no longer competent to make informed decisions. My Moms Medical Directive/POA read like a living will. What she wanted and didn't want was listed in the Directive. The only time I would have needed to make a decision was if something was not covered in the Directive. Thats pretty much is the responsibility of a person holding this POA and its not in effect unless the principle cannot speak for themselves. My Moms read that her Doctor could invoke

You don't say who the principal is (a parent?), how old they are or whether they have cognitive/memory impairment. Just FYI even if this person has some mild cognitive issues, they may still be able to legally assign a new PoA. This person needs to be taken to an elder law attorney who will then privately interview the client for capacity and to make sure they aren't being coerced. Just saying not to make assumptions about this person's ability to reassign PoA. But, if there is a medical diagnosis from a doctor, then they may be too far gone. It's the lawyer who decides capacity, not the doctor. The doctor just diagnoses.

Can't be reassigned by sister. Only the principal can assign POA. However, if you are listed as backup or secondary to sister, I believe your sister can formally resign (in writing) and you could then take over. If the principal (your parent?) is still considered legally competent to re-assign POA, you can take him/her to lawyer and have it done that way.