How hard is it to remove my sister from my mom's medical power of attorney and make me medical power of attorney?

Being POA or MPOA is really not that great. This might be a good thing.....

If you are secondary, then sister could step down and let you take over.

No, she cannot be removed unless your mother is entirely competent. As you posted this under Alzheimer's and Dementia care and as you say Mom is in Memory Care now, it is impossible to change her POA. You would be able to apply as her guardian, but if sister fought that it would be unlikely you would win.
This is the time for you and sis to either pull together or for you to pull back entirely, tell sis that you will allow her to make ALL decisions regarding Mom and tell her that you are happy to be boots on the ground to let her know what you see on visits IF SHE WISHES IT.

There's nothing you can do here.
Your best chance is to become Sis's best friend, so I would advise you to start LYING LIKE A RUG.
It is "Sis, I am so sorry you have to be the decision maker in all this. It has to be a terrible burden. I am right here if you want to talk or have questions I can answer. Otherwise know I trust you and I will let you know if I see anything concerning IF YOU WANT THAT." That's it. No matter how your insides curl like a new-born batch of garter snakes. That's IT.
This is in YOUR best interest but more than that, this is in your mom's best interest.

Sorry that this is how it came out. I truly am.

Perhaps we could make better suggestions if you explained “my sister is being very difficult”. For example, it often means ‘resisting a facility to save money for inheritance’, but that isn’t the case here. What is Sis doing that affects M and you?

Have you discussed this with your mother and sister? Why do you want your sister to relinquish POA? What is not accomplished that you feel you are better suited to be POA?

She can voluntarily relinquish POA if she feels you can meet the needs better and this can be done with letter best drafted by an attorney but not necessary. And, since you are secondary POA it would automatically revert to you without going through the court system as Guardianship would require.

IMHO, a POA should be local or at least not across the country.

Only the principal, your mother, can remove a POA. And she probably cannot do that due to incapacity from dementia. You can always ask an attorney to interview her in person to make that determination, providing your mother is agreeable.