Four people are sitting around a campfire after a long day of recreation, when one man comments: "Do you realize that around this campfire, the four of us include a mother, father, brother, sister, son, daughter, niece, nephew, aunt, uncle and a couple of cousins"?.
If everyone is related by blood (with no unusual marriages) how is this possible?
It's a family of three with a tag-along: A man, a woman, their child (a boy) and the child (a girl) of another couple who are the brother and sister, respectively, of the present man and woman.
So the man is a father to his son, a brother to his brother, and an uncle to his brother's daughter.
The woman is a mother to her son, a sister to her sister, and an aunt to her sister's daughter.
The boy and the girl are a 'couple' of cousins.