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Genealogy

Answer:
Person #1, #2 and #4 are males.
Person #3 is female.
Person #2 is person #3's only son.
Person #1 and #4 are person #3's brothers.

Explanation:
From hints 2 and 4, person #4 can not be a only daughter and have a sister (person #2 or person#3).
Therefore person #4 is person #1's brother.
Person #4 is male.
From hint 4, let's assume that person #2 is person #4's sister. (person #2 is female)
From hint 3, person #1 is person #3's only son. (person #1 is male)
But person #4 is person #1's brother which means that person #1 is not person #3's only son.
Thus, our assumption was wrong.
Therefore, person #3 is person #4's sister. (person #3 is female)
Now person #2 must be person#3's only son.
Persons 2 and #4 are males and person #3 is female.
Person #1 must be a male as only one of them is of opposite sex from each of the other three.
And he is person #3 and #4's brother.

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