first off, just wait a little. there have been a lot of rumors about a Wii price drop that'll go into effect October. if it happens then the Wii will be 200$, thats 50 bucks you get to spend on a game.
and people say the wii gets old, well thats cause they're not getting the right games. you want to play with your sister, well what kind of games would she play with you?
Racing, get Mario Kart Wii
Fighting, get Super Smash Bros Brawl
just anything, get both of them plus any random shovelware crap for good measure.
the Wii also comes with Wiisports.
plus coming soon is New Super Mario Bros Wii. all the classic fun of Mario with up to 4 pkayers at once