My husband has POA for his mom. She and his sister have a joint account. His sister is using funds for personal use. How do we change the account?

Your mother needs to stop putting her money into the account and to take her name off the account. Is she competent? Or will your brother need to do this for her? It's not good to have two older people's names on the account. If Medicaid is needed, all the money in the account would be considered as belonging to the person applying. Also, any debt incurred would be considered as belonging to the person with the debt. Not a good idea. What if her sister was to get sued? All your mother's money would be at risk. Your sister can also spend your mother's money if she chooses. It is a simple thing to no longer put money into the account.