The power of attorney document that your husband signed may give him the right to delegate authority to his sister. Absent that, he may be able to resign as agent and allow the successor agent to handle the responsibility.

If no successor agent is named, his sister must petition the probate court to become the legal conservator (and possibly guardian); at which time your husband can resign.

If the person (I assume a parent) to which your brother has power of attorney is able to create a new document, that option would be best.