I’ve figured it out. Imagine that Mia farrow and woody Allen were cousins and Soon-Yi were the child of Farrow’s deceased previous husband (I assume there’s no divorce, but I’ve never played this game). Then Soon-Yi marries a sibling of Ronan Farrow, who also dies. Now, if she and Ronan farrow get married, she’s his cousin, half-sister, sister-in-law, niece, and wife.
Then if he’s the king and they like each other, she’s the queen, his vassal, and his friend.
Edit: I guess Mia farrow would also have to be dead for Soon-Yi and Allen to have gotten married.
A typical relationship in Crusader Kings III angielski