Okay, your lights are on fron 8am-2pm. That is just 6 hours. They probably should be on for at least 8 hours, if not more depending on your corals. Why not set the timer to come on at 3pm and have them stay on till 11pm? If your brother comes in and they are already on then he will leave it alone, and feel vindicated.
However, as has been touched on, where are the adults? Can't some meeting be had where parameters are going to be set for both him and you to abide by? I know lots of homes have chaos and no parental intervention.
All in all though, your choice is to allow things to go as they are, or find a solution.
Good luck, Bill.