Is it illegal to tape record someone without their knowledge? Does it vary from state to state?

The rules vary from place to place, but in public, people can usually be tape-recorded without their knowledge or consent. There are exceptions, of course., e.g. courtrooms, some types of government hearings, and the list goes on.

In a private place such as your home you can expect that no one will audio record your conversttions, unless the government gets a warrant, and even then the restrictions on time for such recordings exist.

Finally, it depends on the circumstances or the recordings and whether or not those circumstances would constitute a "reasonable expectation of privacy." For example, your brother was recorded giving a public speech. There is nothing illegal about that recording because of the public nature of the place. If the discussion was just between the two in a private place like the home, then that would likely change the rules. Check with a lawyer with details to be sure.

Good luck to you.

NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney that practices in the subject practice discipline and with whom you have an atttorney client relationship along with all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.