Sire ID indicates who your father is. The number is the player ID of your father. Useful for finding out who is related to whom, and who seduced your wife or daughter. When a player character dies, the heir loses its current player ID and takes that of head of the family (the recently departed player character.) When this happens, all the Sire IDs created by the recently deceased character disappear, leaving the way clear for the new heir to make his mark.