I was an emperor of Britannia and a dozen kingdoms. The inheritance low was "Agnatic-Cognatic Primogeniture" (screenshot was taken after the emperor died).
When my Ruler died, he had two daughters. One was the heir and inherited half of the empire.
Somehow the second got literally the half of my kingdoms, including the duchy with the capital.
Question #1
Luckily she is still my vassal, but how did she get the half where the inheritance law was primogeniture?
Question #2
I guess I can cause her to revolt and revoke all kingdoms and the duchy, but I'm afraid counties' revocation may cause a lot of unrest.
What is the best way to get back kingdoms and my main duchy (Mercia, if it makes any difference)?
Best Answer
When a character dies, then all their titles get distributed according to the inheritance laws which apply to each title. Kingdoms within an empire do not necessarily have the same succession laws. It is possible that some of the kingdoms had agnatic-cognatic gravelkind or elective succession and thus went to the younger daughter.
Regarding getting those kingdoms back: Check the lines of succession of the kingdoms. If you murder her and her two children, then the kingdoms should go back to you. If murder is not a viable option, then it might become quite difficult to get those titles back into your demesne. Honestly, the easiest way to unify that empire could be to eradicate your branch of the family so she becomes your heir and then die so you continue playing as her. In order to do that you need to make sure that your two sons die childless and that your husband doesn't knock you up again.