Unless your sibling has been named as an alternate agent in the original power of attorney document, your father is the only one who can change the choice of agent.
Some people who have been diagnosed with dementia are still able to meet this legal standard; others are not. If your father’s condition makes it impossible for him to modify his power of attorney, then there are a couple of other possibilities for you to pursue.
First, explain the situation to the ombudsperson or family representative at the nursing facility. He or she should be able to help you and your sibling understand whether such a legal change is truly necessary—or whether your dad’s daily care needs may be met more informally through the facility.
If the need for some type of legal authorization remains, your sibling may need to obtain an adult guardianship or conservatorship over your father. This could basically entitle him or her to make caretaking decisions and to manage your father’s remaining property and finances. You can obtain more information about requirements and specific procedures from your local probate court. Many of them have established self-help centers that make the process of getting a guardianship or conservatorship quicker, easier and less expensive than they would be is you hire a lawyer to handle the entire procedure.
And to make that change, the law requires that he must be mentally competent—that is, to understand the nature of the document and what it means.