i get terrible nightmares, and my life in real life has been a living nightmare too i have to say. i could go on and on about things that have happened to me that are unbelievable and people around me are afraid to be near me sometimes. they think i am bad luck i think. my nightmares while i'm seeping are horrid and i can't tell if they are real or not. sometimes when i wake up i have to go through it and realize it was a dream. your sister could be causing you nightmares because at some time she scared you in real life, it doesn't have to be anything really really scarry, just something like when you were a kid when things were more scarier that you got over already in your conscious life that you never solved in your real life. i don't know but that may be something to do with it.