I'm not a dream expert, but I often try to interpret my own dreams, and what I've learned is that our dreams tell us a lot about ourselves (our fears and hopes) and the ways our minds work. Sometimes they do this symbolically, metaphorically, or spell it out for us.

A lot of dream interpreters will try to tell you that your dreams are a warning/premonition of what's to come, but that is very unlikely. They are more likely to be things in your subconcious, that you don't openly think about, but you worry about in the back of your mind. The best dream interpreter of your dreams is you, because you know your daily experiences and emotions better than anyone else. You can learn to interpret them over time by keeping a dream journal, and writing down everything you remember the moment you wake up.

The more vivid your dream, the more real it seems, the more significance it will have to you in portraying your hopes or fears. I'm not saying that you want to see your sister run over by a truck, but maybe the dream is expressing that you are worried about your sister. Since she's your younger sister, maybe you're protective of her and are afraid that bad things will happen to her. Again, you are the best interpreter. Write your dream down and maybe it'll start to reveal things to you later on.