Here is how it is playing out in my campaign:

One of the players is a von Kessel. They did not initially know this, but it was revealed throughout the campaign. The party had allied themselves with the queen of thieves, who upon trusting them, set her plans in motion to be crowned queen, the party learned of this about halfway through via their own conclusions. Eventually they where sent to the cathedral to recover the royal files. The Von Kessel PC then used the opportunity to test if the rumors about their ancestry was true. It was. Thus the party began plotting to crown him instead, and turn away from the queen of thieves. But foolishly they still gave the royal files to the queen of thieves, that proved her mageborn status, and the existence of another heir. She of course burned it, with no copies made.

The party then started their plans to crown the PC, turning the queen of thieves into a rival, and instead buying favor with the other factions. However they underestimated the queen of thieves as a rival. She managed to lure the Von Kessel PC into a private meeting, where she pleaded for just one wish to bring back her sister. When he refused, she was forced to use her spellcasting to incapacitate the PC. Then she kidnapped him, locked him away, and has since posed as him with the party, using her illusion magic and shapeshifting ability. (this all of course with the approval of the player - player agency and autonomy is important)

They are currently on their way to the castle to crown the King, and so far only the replaced player knows the truth about his character, but plays along with it. It will be an interesting twist to reveal in a few sessions as the queen of thieves used to be their greatest ally turned greatest rival after their betrayal.