Well, to make things easier, let's just list off what I played when I first got into gaming (I was 3 or 4 at the time, so, at the most, half your sister's age)
the Freddy Fish PC CD-ROM Point-and-click games
the Spy Fox PC CD-ROM Point-and-click games
the I Spy PC CD-ROM Games
Mario Party 2 or 3 (Not sure which one)
Delta Force: Land Warrior
stuff like those Thomas the Tank Engine PC games
Namco Museum (A few years after that later, but still applies)
Tetris Blast
So, yeah, those are the games that got me into gaming. Aside from those, I'd reccomend something like Kirby (Aside from the bosses, the levels tend to be very easy, and the controls are simple), Bomberman (Run, drop bombs, run away, bombs blow up. Easy peasy), or Pokémon (The best thing about the series is that, on the surface, it's simple enough for a relatively inexperienced gamer to get into it fairly quickly, yet it has some deeper gameplay aspects to appeal to more skilled players)
Sorry for any errors that I've made, I was barely awake when I typed this.