introducing a boy, a girl said, he is the son of the daughter of the father of my uncle. how is the boy related to the girl?

The boy is related to the girl as her brother. Explanation:

• The "father of my uncle" is the girl's grandfather.
• The "daughter of the father of my uncle" is the grandfather's daughter, who would be the girl's mother (or aunt).
• The boy is the son of this daughter, meaning the son of the girl's mother (or aunt).
• Since it commonly means the girl shares the same mother, the boy is her brother.

Thus, the boy is the brother of the girl.