No, the Heirs are not the same as through history. The game starts with the correct ruler for each faction, eg William II in 1097 for England.

Then when princes and princesses are born is dependent on age and marriage I suppose. Although according to the manual, if you get a ruler who did have a corresponding "real" person in history, they will have the same traits.

Eg Edward I for England will be a Great Builder, Henry V a Great Warrior.