A lot of people don't realize that establishing paternity is more than just creating a legal link between a father and his child for purposes of setting up a support obligation. Paternity has a number of other useful purposes such as giving a child better access to their family medical history, federal benefits, and building a relationship between father and child that can then be fostered through visitation or custody arrangements.
Imagine that one of your parents was someone who never existed. Sound like science fiction? It's not. Because of a rare genetic blending effect known as a "chimera," it is possible for genetic material from a twin lost in utero to be absorbed into the surviving twin. If those cells become sperm or eggs, then it's possible that those sperm or eggs could go on to create a child whose parent is a twin that was never born. In other words, it's possible that a person's son or daughter could actually be his or her niece or nephew, as was the case for this Washington couple.