Bad Blood
Uncommon12.9%ObjectiveOptionalSingle-player
Go to war with one of your siblings over a Claim
The 867 Carolingians bookmark exists for this - the brother-kings all start with claims on each other's kingdoms. Pick any of them, declare war on a sibling using the claim CB you already hold, and the requirement is satisfied on declaration - you can white-peace out afterwards if you'd rather not actually fight your brother. Very medieval. Same bookmark serves 
