If I am POA for my Mom and have joint saving account with her, can I withdraw funds upon her death?

These are two separate questions. The Power of Attorney expires when the principal dies.  youshould no longer use the Power of Attorney after the person dies.   however, you are the joint bank account holder, therefore you do not need the Power of Attorney to withdraw funds from the accounts.  when a joint bank account holder dies, the asset becomes the property of the surviving owner.