Well, a few first thoughts that I have:
1. Who is your sister that she is able to not let you play certain games? Is she an authority over you? If yes, then God says that you should obey her. If she's not, then you should appeal to your higher authorities -- your parents.
Going against the authority that God has placed in your life is truly more satanic than any video game can ever be.
2. Does the game come between you and God? I don't mean in a sneaky, creepy way where you had no idea it was doing it and it did it anyhow and you didn't realize for years and years. That kind of way doesn't happen, because God is wiser and more powerful than satan. I mean in a direct way.
Let me illustrate from my own life: Sometimes, I play fighting games like Street Fighter, Mortal Combat, or Soul Calibur. When I do, I have to be careful how much I play, because I have a tendency to get over-obsessed with combo moves. When this happens, I will spend all of my time with combo moves running through my head. I won't be able to sleep. I won't be able to focus on anything but "if the enemy uses this move, press B, down, A, and up" and so on and so forth.
And that blows my focus. I can't pray. I can't think about God. All I can think of are combo moves.
That's bad. I need to be able to pray, to focus. So fighting games need to be controlled in my life.
Straight out, this kind of badness can come from any game or activity, whether it's Runes of Magic, Mortal Combat, or Bejewelled.
