If your brother needs the free night, the ressie needs to in his name, his MR # & he pays for it. Have him add your name to the reservation though (they won't be able to put in your #). If you're going to check in with him, you can (nicely) ask if they'll extend gold bennies as a courtesy. They're not required to do so. If you're lucky they'll say yes. If you're not, then enjoy the stay anyway & know that your brother will get a free night after the stay.

Cheers.