Florida Power of Attorney from the Florida Bar Association is a consumer pamphlet summarizing the laws in regard to a power of attorney. The actual laws are in the Florida Code sections 709.2100 thru 709.2402
Health Care Advance Directives from the Florida Health Care Administration is a consumer guide to Living wills, surrogate designations, and related issues.
Florida Living Wills Laws from FindLaw lists legal provisions related to living wills in Florida. The relevant code sections are stated as 765.101, and subsequent sections (Health Care Advance Directives).
The person who makes the Power of Attorney (POA) is the principal -- the mother in the situation described in the question. The person appointed to do things for the principal is known as the agent, and is sometimes also called the attorney-in-fact.
A Limited POA gives the agent power only over a particular transaction, or over a single or small number of types of transactions. One might give a limited POA to manage a particular investment, or to sell a car or a house, say.
A General POA gives the agent power to do anything that the principal could do in person, or anything except for some specifically excluded things.
A Durable POA remains in force even if the principal becomes incapacitated (for example with serious dementia). It includes specific language to this effect. It is usually also a general POA. Any POA that is not durable is not in effect when the principal becomes incapacitated. The situation described in the question would often be met with a durable POA.
A durable POA:
contains the words: “This durable power of attorney is not terminated by subsequent incapacity of the principal except as provided in chapter 709, Florida Statutes,” or similar words that show the principal’s intent that the authority conferred is exercisable notwithstanding the principal’s subsequent incapacity. (Section 709.2104)
A Medical POA, sometimes also called a Healthcare POA grants the agent power to make medical or health choices for the principal. It is also known as a health care surrogate designation. It is normally a separate document form a POA dealing with non-medical issues, sometimes called a financial POA, even if the same agent is appointed for each.
A living will is "a written or oral statement of the kind of medical care you want or do not want if you become unable to make your own decisions." (from the Advance Directives pamphlet linked above)
Living wills and Healthcare POAs are collectively known as Advance Directives. The pamphlet says:
[I]f you have not made an advance directive, decisions about your health care or an anatomical donation may be made for you by a court-appointed guardian, your wife or husband, your adult child, your parent, your adult sibling, an adult relative, or a close friend.
Under section 709.2102:
“Incapacity” means the inability of an individual to take those actions necessary to obtain, administer, and dispose of real and personal property, intangible property, business property, benefits, and income.
Any competent adult that the principal selects may be an agent. The pamphlet linked above says:
Any competent person 18 years of age or older may serve as an agent. Agents should be chosen for reliability and trustworthiness. Certain financial institutions with trust powers also may serve as agents.
The agent may hire assistants, such as a lawyer,, an accountant, or a realtor. But the agent may not delegate or transfer his or her powers as an agent, or appoint a sub-agent.
The agent must act as instructed by the principal. When no specific instructions apply, the agent must act in the best interests of the principal, to the best of his or her understanding and ability.
Creating a POA
The POA must be in writing, and signed by the principal and two witnesses, and notarized. The principal must be competent when the POA is signed -- that is, must be in condition to understand the purpose and effect of the POA, and what is going on generally. (See the definitions section above.)
The agent must accept the POA, and often signs the POA to acknowledge the POA and accept the responsibility of acting as agent.
A POA may appoint backup agents to act if the first agent is unable to act, or resigns, or dies. It may also appoint multiple agents any of whom may act, but this can cause problems if the agents disagree.
The text of a POA may be drafted by a lawyer. But there are also software programs that will produce the text of a POA, after appropriate information is entered and questions are answered as to the desired effects. Such programs normally produce text tailored to ma particular state's laws.
Ending a POA
The principal may revoke a POA at any time, if the principal is competent. A principal may also revoke a living will or other advance directive at any time.
The agent may resign at any time
If either agent or principal dies the POA is ended. (If a POA appoints multiple agents, it ends only if they all die or resign.)
If the agent becomes incapacitated, the agents power ends. If there is not other agent, the POA ends.
If the agent is married to the principal, a divorce or separation or a petition for a divorce, ends the agent's authority, unless the POA explicitly says it will continue after a divorce or separation.
If a legal process to appoint a guardian is started, the POA is suspended, unless the agent is the principal's "parent, spouse, child, or grandchild". (This may include stepchild.) If a guardian is appointed, the court will rule on whether the POA continues; often it will not, unless the agent is also the guardian appointed.
Florida code section 709.2109 (3) (Link above) says:
If any person initiates judicial proceedings to determine the principal’s incapacity or for the appointment of a guardian advocate, the authority granted under the power of attorney is suspended until the petition is dismissed or withdrawn or the court enters an order authorizing the agent to exercise one or more powers granted under the power of attorney. However, if the agent named in the power of attorney is the principal’s parent, spouse, child, or grandchild, the authority under the power of attorney is not suspended unless a verified motion in accordance with s. 744.3203 is also filed.
If the POA includes an ending date or condition, it ends when this occurs.
A third party, one who is neither the agent or the principal, can only have a durable POA revoked by petitioning a court to do so, usually in connection with the appointment of a guardian. The court will do this only if a guardian is needed, and that the action of the agent under the POA is not sufficient to serve he interests of the principal; or if the agent is shown to have been untrustworthy or neglectful, or has become incapacitated.
Living wills normally remain in effect unless revoked, even if a guardian is appointed.
No one has an automatic right to be appointed as the agent, no matter what relationship to the principal that person has, or for how long a relationship has existed. The principal may choose any agent that s/he trusts and thinks will be able to do the job well.
The husband or stepchildren described in the question will not be able to have the POA set aside by a court unless they can show that the son has not acted in the mother's best interests or has acted dishonestly in regard to the mother, or has misused the powers granted by the POA. If they can show that the son used undue influence to get the mother to sign the POA, that might cause a curt to rule against the POA. But they have no automatic claim to cancel the POA, just because they are related to the mother.
The Florida laws about a POA are very similar to the laws in NJ and NY that I have had occasion to deal with before. I held a durable POA and a healthcare POA for my father, who lived in NY state.