I don't think there is anything to feel guilty about for roleplaying a sexual scenario when you were younger, even a rape one, considering that there was no real sexual feelings or molestation involved.

I think a lot of us have stories of some embarrassingly 'dark' things we thought and did as children. You grew up in a world surrounded by portrayal of sex. There is no way to keep little kids from knowing about it! Roleplaying sex as a little kid isn't twisted, it's part of understanding your world.

I think your sister is A-OK and I don't think you should feel guilty about it.