Mom just asked if I would call her bank to see how much money she has. Since I’m not the main POA and just an alternate, can I still call?

You never have to accept POA if you don't want it. Inform the lawyer, and you don't need to give any reason.

With the bank, if your mom can identify herself on the phone when you call them, and if she can tell them that they have permission to talk to you, they'll probably do it. You'll need her account number and other identifying features, such as SS# and her address. That's worked for me in the past. Or tell them you're putting the phone on speaker so you can both hear it (mom's hard of hearing, right?).

"It is a good idea to have an alternate power of attorney. This person will make decisions for you if your first choice is no longer available or does not want to act as your power of attorney any longer. They will only act as your POA if your first choice is not willing or able to do so"

You can contact the lawyer who drew up the DPOA and ask that your name be removed. Or, if Mom is with it. Write a letter stating that you do not want this responsibility and you are stepping down. Have the letter notarized. Give a copy to Mom and your sister. If u know who the lawyer is, send him a copy. If Mom did not ask you before she made you an alternate, then put that in the letter saying you never excepted the assignment.

Just be aware, that if Mom has been found incompetent, that if something happens to sister Mom has no representative. The way Sis handled the money will have no bearing on you but like u said, u will need to clean up the mess.

I think your sis gave mom the perfect answer.

I would walk away from this mess. Let sis deal with mom.