Answer:
There are no IRS rules on this subject.
A court appointed guardian or conservator may be empowered to take care of the person, or the property, or both. What a person empowered to deal with the property is allowed to do is dictated by:
* the terms of the order appointing him or her and
* state law, which is usually quite detailed.
Read the former. Check with your bank's legal counsel on the latter.