Enact a subrule to “Monarchy” entitled “Seizing the Crown”:

If the crown is currently unoccupied, any noble may take the crown by force by expending 30 of their knights. This number is reduced by 1 per point of influence they have in Training Grounds.
If the crown is occupied, any noble may reduce or increase the total number of knights of the noble who has taken the crown per the rules in “Acts of War”.
It is outrageous for a noble holding the crown to not host a tournament at least once per time advancement. (This only requires proposing a tournament, but does not depend on people actually participating in the tournament).
If any of the following are true the noble who has the crown loses the crown. This check occurs before any other (including victory):

• the Noble holding the crown has no knights
• the Noble holding the crown has less than 50 dignity

Holding of the crown is tracked at the top of the peerage wiki.
It is improper for a noble to harm the noble holding the crown if they are members of the same family.