Pedro, Paco and Pablo are three married men and none have more than one sister. They are waiting for Pablo's father-in-law and Pedro's father to go to the movies. Pedro and Paco are Pablo's brothers-in-law.
One of the three is an only child. Which one is it?
Pedro is an only child and is married to Pablo's sister. Pablo's father-in-law is Paco's father, since Pablo is married to Paco's sister.