I have a casus belli, but I can not declare war because no hostages held

I am playing as the count of Breifne in Ireland. My chancellor has fabricated a claim on the neighboring County of Connachta. The count of Connachta is also the petty king (duke) of Connachta, although holding only this county.

My idea was to now somehow press the fabricated claim, but I don't know how to do this. I can not declare war on the petty king. The tooltip over the declare war button says that I am fullfilling all requirements (including the causes belli) but one: The petty king "does not hold any of your close relatives hostage".

How can I press my claim and declare war on the county of Connachta?

Solution 1:

Holding close relatives hostage means that that character is educating one or more of your close relatives at his court, or has one of them as a prisoner.

Check who is educating your children and any young brothers and sisters. If they are normally part of your court (they probably will be), make someone else educate them. This will annoy the person who was educating them but this shouldn't matter- you're about to go to war anyway.

Check if any of your relatives are imprisoned too and ransom them if possible.