To boil your question down, yes, a guardianship will replace a power of attorney executed by your step-father. In fact, to ensure that an agent given authority under a power of attorney is properly given the chance to defend that authority, agents under a power of attorney must be provided with specific notice of any guardianship application. You're also right that your half-brother is lying to you if he hasn't been to court.
This is a pretty critical time in your step-father's life, and while I'm sorry that it's at the center of so much family turmoil, you need to be proactive. Start searching for guardianship attorneys now. This is a fight just waiting to find its way to the probate court.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship.