Your lucky as hell to have a sister like that. Some people are just naturally better at some things.
I had a friend years ago that was like her. I found out I was REALLY good at Atari asteroids. After he saw I could line up free ships all the way across the screen and he couldn't get past 50,000 points he settled down.
Just find something you can naturally whip her at and when she sees that all the "competition" will be gone.