Unfortunately, this next party is on the 9th, and after deejaying the 7th and 8th, I want a day off before going to the DJ Expo on the 10th, so I am putting my brother on it. He can use the money anyway, and I think it will be good for her guests and her to see another DJ from my company.
You didn't ask but I'll say it anyway, I think you're making a mistake. This client is a repeat and wants YOU, not your brother. If DJ Expo is costing you money then it's counter-productive for you. Personally, I'd just man up and work the job after all, it's a simple casual.