depending on your brother/sisters view on video game violence I give you what's cool right now. Unfortuanly a lot the popular games for that age are on the game boy advance and game cube. You're not going to find much for someone that young on the PC, there are some games but the PC market isn't as geared twordsa that age group like consoles.
If he likes Dragon ball (and his parents wouldn't care about a beat em up game), Dragon Ball Z: Budokai 2 comes out the second of December. The first one was popular and easy to play.
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Eye Toy is going to be realy hot this christmass. it had a huge selling in Japan, Europe and the US. It basicaly is a camera that puts you into different mini-games.
You can also never go wrong with a Tony Hawk game.
Also there are several types of raceing games depending on what he likes (cars, dirt bikes, jet skies) and kids that age like those games.
A new TMNT game is out that plays like the old arcade games (ie Fianal Fight). So if he's a fan of the cartoon that's a good way to go.
And if he's a snowbording fan SSX3 is a great game.