>>1's a lot more mature-sounding than many of the supposed 18+ people on this board, so stop telling him to "lurk moar". (Although not knowing what a troll is might mean that he does need to read up a bit on board terminology. BTW, a troll is someone who's just posting nonsense outside of the nonsense (DQN or /b/) board.)

As for the actual topic at hand, my advice is to just refuse to be alone with the squirt. Tell your parents that you're not going to babysit her anymore because you legitamitely feel that she might hurt you and that even though you are bigger than she is, no one's taught her not to stab people with knives and you fear for your life.

When your parents argue the point, stand your ground. Above all, just say that you refuse to be in a situation in which there is not an adult present when she is with you. If you ever do find yourself alone with her, just go in your room, lock the door, and come out every so often just to make sure she hasn't slit her wrists or something. (After all, she IS your sisiter, and you don't want her to go balistic and off herself instead of you.)

I don't suggest slapping her, just because I've never seen violence really solve anything. Then again, I'm a girl, so I haven't had much use for violence anyway. ^_^ I guess guys have different ways of dealing with things.