Look, dood.
Your mind is trying to wrestle with something- but it can't just say "you're having problems with _____ on a subconscious level". Dreams are typically more about significance than they are about the actual events that occur in them. The one with your sister might indicate that you feel like she's being taken away from you or taken advantage of in some way- but because your brain is using the imagery it knows, it like- goes about it by trying to make symbols of things.
Idk if that makes sense. But I've had enough terrible dreams- like, I'll be talking to someone and someone else will come along and shoot them in the face, or just other heinous crap. I've even had dreams about having sex with people I wouldn't touch in a million years.
Point is, just because the imagery might be scary or gross, your brain is trying to solve a problem by using references. I'd try to cut down on any horror shows or news stories, if you can. And if all else fails, hypnotherapy and hypnosis are always valid options.