What makes you think Brother wasn't smart before? Think about it, he's mid-way through learning a language and probably still dosn't know what half the things he's saying actually mean. Plus he's in love with Yuna, therefore making him a bit...silly (its happens to people when they are in love). Makes sense in a weird way. So basically he's acting the way he is out of confusion I guess.....not because he's lacking the smart gene. He's definatly the weird one though, lucky Buddy and Shinra are there to keep him in line.