What does it mean when you dream a sibling dies?

Dreams are just dreams and do not "mean" anything.

Addition to the previous answer:

While they may not "mean" anything persay, it can be a pretty accurate indicator of what's going on in your head, realizing, of course, that you can't take dreams at face value, because while they seem connected, and one big dream, they usually aren't. The indicators of what's going on in your psyche is more about colors that appear in the dream, and other seemingly irrelevant things.

... A dream about death usually indicates a new chapter in your life, death meaning the end of something, not particularly someone dieing. The fact that it was your sibling, may be totally unrelated, or, it's possible that your relationship with your sibling reached a new level, be it good or bad.

While this all seems like witch craft, type stuff to some people, it's not. Your brain can only hold so much, and has to purge it somehow, because it does get filled up, like a computer, and can't hold anymore. You'll find the brain purging via dream, or out of place emotions.

So, in regards to dreaming, the sibling, and the death, very well may not be related. Usually, anything dealing with a psychological answer, isn't near as cut and dry as I think you were hoping for.

*Please note, this is coming from a Psychology student. I'm not, nor am I trying to put myself out to be a licensed psychologist. This information just comes from a culmination of what I learned to this point in my schooling.*