I'm considering theoretical possibility of using the following scenario to obtain more land for my heir by successfully marrying my other children and then engineering the succession (murdering everyone).
I'm John of Jones dynasty. I have two children - Gary, who is my firstborn son and Lisa, his younger sister. I'm using Agnatic-cognatic Primogeniture. Gary is a genius, so I definitely want him to be my heir.
My neighbour is Ted of Smith dynasty. He also has three children - Alan, Bob and Chris and uses A-C Primogeniture too. He is a duke, so he does not have any Crown Authority.
Now let's say that Bob hates his dad and accepts my invitation to court. I arrange a matrilinear marriage between Lisa Jones and Bob Smith. They produce a son, Nick Jones.
In the meantime I kill Alan Smith, so that Bob becomes his father's primary heir. Soon after, Ted Smith dies too and my son-in-law Bob inherits. His heir is now Nick Jones. So far so good!
Let's suppose that Nick dies before he has any children of his own or brothers/sisters. Who would be Nick Jones' heir? Would the heir be Gary Jones (his mother's brother, of his dynasty) or Chris Smith (his father's and previous liege brother)? Could Gary inherit both my Jones kingdom and his nephew's Smith duchy?
Family tree: X - dead, == - marriage XTed SmithX | ----------------------- XJoe JonesX | | | | XAlan SmithX | Chris Smith ---------------- | | | | Gary Jones Lisa Jones ============= XBob SmithX | Nick Jones