A Family Reunion

[The problem was taken from http://public-library.calgary.ab.ca/srg/gamesw8.htm . That site indicates that it was taken from Paul Sloane's Lateral Thinking Puzzlers.]

At a family reunion, it was found that the following relationships existed between the people present: Father, Mother, Son, Daughter, Uncle, Aunt, Brother, Sister, Cousin, Niece and Nephew. However, there were only four people present. How can this be so?