You should ask yourself, although you are now an agent, you are new. Are you doing this just for the money? Or do you really think you have something of value to add to help your brother? I would definitely run everything by your broker and use this as a teaching experience.
Good luck and feel free to call to chat!
The original agent may be due the commission since they may be considered the procuring cause for the purchase. (They showed the property to your brother). You might want to talk to the listing agent - maybe you could work out where you could pay them a referal fee if your brother's offer is accepted with you acting as his agent?