Can I transfer money to an account in my name only so I can budget it for my sister?

I managed to sell my sister's house and find her a comfortable independent living situation.  Control of finances for a sister with declining abilities to manage her money. My 74 y.o. sister was a hoarder - I managed to sell her house and find her a comfortable independent living situation with her 43 y.o. daughter. The money is in a joint account with me. I also have POA. This money from the house sale was to be for her rent and living. She is spending the money frivolously and her kids are also taking advantage of this "windfall". The question is: can I transfer this money to an account in my name only so I can budget it for her? Her mental state is declining but she is still lucid. She is unwilling to get a medical assessment. This is her money but she simply doesn't understand that it's need for her rent, bills, etc.