My sister has POA for my mother who has Alzheimer's. My sister has recently been diagnosed with the same disease. What do I do?

Agree with posters - my dad's POA lists mom, then me, then younger sister.  I recall there being a line "if unable or unwilling to perform responsibilities".
Mom's POA lists me then sis.
Mine lists Mom then sis. I need to add a third, but I need to do more research about passing inheritance to my (minor) son via trust, before talking to one of his "uncles". Once my son turns 18, I'll put him down as first or second.

There is usually a second person on the paper incase somehting happens to primary POA .

Have you seen the POA document? Sometimes there are provisions naming a second person to step in if the primary person named can no longer serve.

Sorry, only your Mom can direct Power of Attorney to someone else. Unless on your Mom POA she already had two people listed, your sister and a backup person.

This being said, make sure both you and your sister have your own Power of Attorney for future use.