My dad has signs of dementia. My mom has some medical issues, but she is fully cognizant. My sister took control over my dad's finances, putting herself on all his accounts. She refuses to follow the financial wishes of my parents. My mom has asked my sister to return all credit cards, bank account information, check books etc., but my sister refuses. This is causing my mom a great deal of stress, affecting her health. How can she regain control over my dad's finances?
Your mother, of course, can manage her own finances as she sees fit. But she has no particular right to "regain" control over your father's money unless that is what he wanted; and it seems he didn't.
When you say that your sister "refuses to follow the financial wishes of my parents" - what do you mean? If there is a dispute about some particular issue, would it be possible for you to help resolve the dispute?