Your dream brother is getting married, but you are actually thinking about your future marriage.
The brother in the dream is your own incarnation, and he represents your mature and sensible self.
So, your brother's date, the man you met for the first time, happened to be five years older than you.
Go and propose to your cousin who represents your ignorance.
Mom said my cousin was too young to be polite, but in fact it was because you thought you were too young to notice some worldly things.
Go to her house with your wife to propose. Your wife is the person who introduced you to that man in reality.
So, judging from this paragraph, you seem to have some nostalgia for that relationship.
But turning yourself into a brother's dream also means that you don't want to face this problem yourself. Maybe you will deny that you are still thinking about blind date.