Final answer:
The name 'Alexander' should be capitalized as it is a proper noun, which refers to a specific person.
Step-by-step explanation:
The sentence should be rewritten as:
I visited my brother, Alexander.
The name 'Alexander' should be capitalized as it is a proper noun, which refers to a specific person. In English, we always capitalize proper nouns, such as names of people, places, and organizations.
Learn more about Capitalizing names of people