First, english is not my first language and your grammar makes it a little complicated for me to understand everything you are saying, so please reread everytime you write something.
About the dream, I don't think Slenderman has a brother. If you mean like Trenderman or Offenderman, they are not from the mythos, I think they appeared in the Slenderverse by some fans. Also, I don't think you should worry that much for a dream, and if you are intrigued and want to know more about it, you should go see a person who knows about that (dreams and psychology). The guy in your dream doesn't have to be necesarrily Slenderman. Dreams are very symbolic, so it could have some references to Slendy because, for example, you were thinking about him lately. But the other characteristics of the man from your dream and/or the dream at all doesn't really have to involve Slenderman, but instead other aspects of your life.