[ 4 days ago ] The son of my father, but not my brother. Who is he?
If a person is the son of the speaker's father but is not the speaker's brother, then that person is the speaker's nephew. A nephew is the son of a person's sibling, so if the speaker's father has a son who is not the speaker's brother, that person is the speaker's nephew. For example, if the speaker has a brother named John and John has a son named David, then David is the speaker's nephew. He is the son of the speaker's father, but not the speaker's brother.
[today] The son of my father, but not my brother. Who is he?
The son of your father but not your brother would be your half-brother. A half-brother is a brother who shares only one parent with you, rather than both. For example, if your father had a child with another woman before he had you, that child would be your half-brother.