In Relation puzzle type, mutual blood relations or other information of more than two persons are mentioned and information about any two is mentioned.
A man and his wife have three sons, and every of the sons has a sister. How many are there in the family ?
There is only 1 girl since it says that every sons has 'a' sister, if there's only 1 girl. she will be a sister to all the three sons.
so 3 sons + 1 sister + both the parents = 6 members in the family.