Dreams have a lot to do with processing and categorizing experiences and information in your life, but they can also be quite random and downright unsettling.

For instance, my sister and I's relationship was always very bad but a few years ago started becoming good for the first time, ever. I still didn't trust her very much, but I was coming to like her more. One night I had a dream that we had sex. Very horrible. I never told anyone.

I was liking our developing relationship, and we were beginning to share a lot and hang out and such. Deep down, I found it to be very odd because it wasn't my norm with her or any girl at all (to step into 'good friend' boundaries) but I didn't actually 'think' of that.
So now, I think maybe the dream, for me, meant that I liked our developing relationship but I didn't understand where the relationship was going.

There's so many interpretations for a dream like that. Common interpretations would be along the lines of "Something in your life is like what happened in the dream." "You desire something like this either directly or in similar form." "You have different feelings that need to be expressed", and so on with the like.

But none of that is guaranteed to be the even remotely the case or the answer for you. I find dream interpretations can be rather fickle, but you can find your own meanings from them.