The next time that bitch tries to pull out a knife give her two black eyes, and if that doesn't work get CPS or the police involved. If you're just going to say "Well she IS my sister ^_^" when she tries to stab you, you're begging to be killed. I don't think your sister has any mental problems, I think she's just a whiny little bitch who needs some discipline. Maybe a stay in a mental hospital and a few shots in the butt would make her quit being such a little shit.