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 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.