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.
Luckily she is still my vassal, but how did she get the half where the inheritance law was primogeniture?
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)?