Sore Loser.

I just don't get it. Zach has always played so well with others, and he loves when others play games with him. We've always taught him to take losing in stride and to not be overly boastful when he wins. Well, I took the time to play Wii with him yesterday (not something I do often, its more of a Zach and John thing) and the boy has become the biggest sore loser/winner. If he would start to lose at the game, he would quit the game and start a new one. If he won he would be obnoxiously boastful about it. I know he didn't get that from John and I. I'm about 90% positive he picked it up from my little brother, and while we work and work on trying to get him not to act like that it just gets ruined each time he goes back to my mom's house. I hate that he picks up all these bad habits from my brother, but what can I do? I can't punish my mom and step-dad by not letting Zach come over because of my brother, but each time he comes back its worse. Its not just the sore loser/winner thing either. Its everything. My little brother has the worst manners and a horribke attitude and its rubbing off on Zach. We've told him he can't go over for the next two weeks, which makes us feel horrible because its not his fault, but what else can we do?