The irrevocable powers of attorney you describe that give a person absolute authority to deal with another's property are rare.
Courts generally require that the person appointed to act
must have an interest in some underlying property, so that if the power were revoked he or she "would be deprived of a substantial right." In real life, that means that such documents are usually confined to stock transfers and such in which those involved are related by blood or business. However, if you were likely to be legally entitled to take your stepfather’s property if he and your brother died, you may fit this legal description.
You could also consider going to court and petition to have you or another person become your brother's conservator, also called an adult guardian in some states. This legal proceeding would make you responsible for your brother's finances and prohibit your brother from regaining control of his affairs without the
consent of the court.
While this step is rather drastic, it may be the best way to provide complete protection. It will, however, mean presenting the court with evidence of how and why your brother needs such protection, which may be painful or embarrassing to both you and your brother. To find out more about the local procedures required to get a conservatorship or guardianship in place, contact the Florida court closest to where your brother now lives.