Plus, if the story is not an official legacy or dynasty, the player may be using mods where it is easy to find out who the parents of a particular sim are.
Using the standard game, it would be as the others mentioned: you can find connections through your sim's family tree or when another sim enters your household. The last name will probably reveal one of the parents, and sometimes the sim's appearance is a dead giveaway. (For example there is no questioning any child of Juan Darer, but others are not so painfully obvious.)