There are two crucial things to remember about dreams.
1. EVERYTHING in a dream represents an aspect of yourself. The actual objects in the dream are simply whatever thoughts are "on top of the mind" being used in a purely metaphorical way to tell you about yourself.
2. The feelings and subjective personal opinions felt during the observation of your dream are the most important aspect of your dream. Since these feelings are entirely subjective (one person's nightmare is another's paradise) only the dreamer herself is capable of correctly interpreting the dream.
Your brother represents an aspect of yourself. What specifically this aspect of yourself it is, no one can tell you, and you yourself likely don't recognize it for what it is. The dream is trying to bring this aspect of yourself to conscious awareness.
Every dream has a goal and a meaning. In every case, the goal is to bring to light an unseen aspect of the self. The dream-master uses arcane, strange, convoluted and highly subjected methods to communicate with the dreamer because it is not easy to explain to someone, "There is a part of you that won't let go of this event that happened and is unhealthily clinging to this particular emotion in this way. You need to release this tension by doing such and such."
When words fail, visual-emotional metaphor steps in.
There is definitely no need to fear for your brother. The dream is not about him - he's just a convenient metaphor to tell you about yourself. It may seem unromantic to be told that dreams have nothing to do with other people - that you aren't dreaming of anything "out there" at all - but countless observation by many many people has shown this to be true. (And from my own dream-work I'm totally convinced this is the case.)
Once you make the intention to remember your dreams more, and then start recording them immediately when you wake up, you'll find that people you know are constantly dying in them and that every night the dream-master uses some new metaphor to try and express the same concept to you.
So to help you discover the meaning of one dream you can simply record and observe a week's worth of dreams - each dream is likely pointing to the same primary teaching. A teaching that will have multiple layers of relevancy to your life but will have one primary theme that reflects on your own spiritual work in the present.
I guess my answer went far beyond your question but perhaps this intense dream can encourage you to consciously do more dream work. Most people do dream most nights, but simply forget their dreams because they either don't record them immediately (this is essential - by the time 5 minutes has passed you will forget a solid 90% of dream material) or simply don't put out the intention to remember them at all. If you don't put out this intention you'll just wake up and think you didn't dream.
Suffice to say, don't fear for your brother. I've recorded many of my dreams and so many people have died. I've killed people, people have killed me, various monsters have killed me and others, calamities have killed loads of people... Everyone I've ever known has probably died at least a few times.
Death can be a useful metaphor.