A Couple Of Suggestions...
Hey, Manji...
When my friends and I get together, it's all about the X-box. As there's three of us, the number of games that we can all enjoy together are limited.
Lately, we've been working our way through D & D: Heroes, a multi-player co-op which allows for up to four gamers. We also go head-to-head in Crimson Skies, which is quite cool, though it doesn't allow you to create your own profile and customize your plane, which kind of sucks. Still, thumbs up.
I have a personal preference for racing games (i.e. Project Gotham Racing), but I can't seem to get my friends interested. Conversely, they like the sports games, but I'm not a fan myself. In either case, such games allow for head-to-head or co-op play, and provide A.I. to make things interesting. Perfect for a two-player scenario.
As far as personal preferences go, I'd have to say Halo and Batman: Rise of Sin Tzu.
Halo allows for two-player co-op play, which is good, because I can't imagine doing that campaign solo. Freakin' tough, man! The head-to-head is pretty customizable, offering a broad range of maps on which to play, not to mention a slew of modes. My friends and I tend to gravitate towards Slayer and King Of The Hill, but I think these'd be boring for just two players. If you go head-to-head, take the extra time to create a personal profile and customize the level to allow for your own personal preference in weapons and vehicles. It's worth the few minutes it'll take to get things just the way you want them.
Depending on how young your cuz is, you might decide to pass on Halo, though. It isn't gory, but it's still a shooter, which may not be appropriate for all ages.
Batman: Rise of Sin Tzu, based on the Batman animated series, might be a better choice for a younger player. It has a two-player co-op mode that allows you to choose from four characters: Batman, Robin, Nightwing, and Batgirl.
The game-play isn't overly complex, but it'll still provide a challenge for a more seasoned player. As you advance from level to level, you're able to put points you've earned towards the acquisition of new specialty moves, allowing you to customize your character beyond the standard punch-kick-jump variation. It's the kind of touch that increases replayability, allowing you to go back to the game's beginning and see how things would shake out with a different character, or with the same character and a new combination of moves.
And, hey...it's Batman; you can't go wrong, fanboy. And speaking of fanboys, here's an interesting fanboy fact: did you know that the character of Sin Tzu, who was designed exclusively for this game, was created by Jim Lee?
Anyway, Manji. I hope these suggestions helped, and I hope you and your cousin have a good time this weekend...