Both legal arrangements"”naming an agent in a power of attorney and setting up a legal guardianship"”accomplish basically the same thing: They authorize a person to manage and control another person's
finances and sometimes medical care for another person.
Which one you or your sister seek may depend more on your father's condition and mental state than anything else.
Guardianships are more difficult to secure. They require petitioning a court and pleading and proving that the person for whom you seek a guardianship is no longer able to care for his or her own affairs. Close relatives are notified and able to come to court to support or contest the arrangement. And it can be somewhat embarrassing for some people, as it is a public forum and an admission of some basic inabilities that some would rather keep private.
A power of attorney, on the other hand, is easier to secure"”and require only completing and filing a few forms with the local court. The catch may be that to get one, your father would have to agree and participate"”and he can legally do this only if he has the required mental capacity. In the eyes of the law, this means he must understand the nature, scope and effect of the document.
Both a power of attorney and guardianship end with the death of the person for whom they were secured. Your father would need to need an executor in a will or a trustee in a trust to have power to manage his money and other property after his death.