Dreams are important and typically happening to help us in some way. The way to understand the dream is to ask yourself what is/are the essential feature(s) in the dream, why is it happening now, and if the dream is trying to help — what is it trying to get you to become aware of?
Only you can interpret this accurately, but what jumps out from the content of the dream is the issue of similar but different. Is there someone in your life, a situation, an encounter where you are experiencing someone one way, but hoping they do not act in another? The fact that your (good) grandfather is symbolizing the (bad) uncle may be a clue. More specifically, is there a good man in your life that you may be concerned about? This could be that the situation is good and you worry how long it will last, or that you have uncertainty about a relationship with a man.
Finally, since the phrase you use is actually: “don’t be like your brother,” maybe to look specifically at brothers you know in your life.
Finally, keep a journal of your important dreams as they will often give you clues about what the theme is to become aware of.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral