My sister (works fulltime) lives with my mother and uses mom's credit card to buy everything for herself. How can I stop this misuse of mom's money?

If your mom is still capable of making her own decisions, all you can do is make sure she knows that your sister using her credit cards. If your mom knows it and is still legally capable, that's the end of it. If your mom wants to stop it, she just needs to cancel the cards.

If I understand correctly, your sister works fulltime so she has an income of $35,000/year (for discussion sake) and does NOT pay any rent or utilities or cellphone or cable/dish TV bills; thus she has that $35,000/year to do whatever she wants with it.  How do you know that your sister is not paying for any utilities--are you looking at your Mom's checking account every month?  Are you, your Mom and your Sister all co-owners of Mom's checking account?  How do you know that it is your Mom's money and not your sister's money that is/was used to purchase towels, rugs, kitchen appliances, accessories?  Your sister needs to spend her money on something, why not use it to update the house that your Mom and she live in?  What has your Mom said about your sister's spending habits?   What PROOF and EVIDENCE do you have that your sister is "misusing your Mom's money"  and "preying on the elderly"?  We need more background information to give suggestions that are appropriate to your situation.

Mom needs to cancel the credit card.

Does anyone have POA?

Why is sis living with mom?