If your brother has no right, title or interest in the property that is being rented the rental company has a contract with someone who is not authorized to contract for the rental.
This is a general answer only and you should seek the advice of counsel to address facts specific to your circumstances.
If your brother is a property manager for your Father [via a written agreement with your Father], or has power of attorney or some other authority, of which you may not be aware, he would be able to contract for your Father.
As a general rule, one who is not on title has not right to lease the property to third parties.
However, one can act as the agent for the actual owner. Generally, you would want a power of attorney to do this. DId your father approve of the listing with the broker? If not, your father should send a letter informing the broker that the brother does not own the property and he was not authorized by your father to list the property for rental.
If this answer was helpful, please mark it as helpful or as a best answer. This answer is for general education purposes only. It neither creates an attorney-client relationship nor provides legal guidance or advice. The answer is based on the limited information provided and the answer might be different had additional information been provided. You should consult an attorney.
Although you provide few facts, there may be other issues at play here. Is your father elderly? Does he reside in the home? Has your brother been controlling your father's finances? Who will the rental payments go to? If your father did not authorize your brother to rent the house, I would be suspicious about any other financial dealings between them. You may want to do some investigating. If there are irregularities, you should contact Adult Protective Services in your area or consult with an elder law attorney.
My answers to questions posted on Avvo are not intended to constitute legal advice or to create an attorney-client relationship.