First of all, dreaming about my younger brother refers to the characters that need care and attention, not necessarily the younger brother himself. Taking drugs is not good, but it may refer to some changes. Because in the traditional sense, all diets symbolize changes in your body. So taking drugs means that the character has developed bad changes and habits. Filling in a check may indicate something related to the economy. It may also be such a quantity, indicating how much it has paid for bad habits. And you don't know this number, which is a portrayal of how much influence this bad change has brought to the role.
Dream interpretation needs some so-called reference life background events, otherwise it is difficult to explain it independently.
Dreams are a panoramic description of your present situation by the brain.
-Contemporary Duke of Zhou Xiao